Australian rainfall during El Niño and La Niña events

Summary

A late-forming, weak La Niña formed in the tropical Pacific in September 2024, decaying in March 2025. The monthly relative-Niño3.4 index peaked in magnitude in January 2025 at a value of −1.36 °C.

Most atmospheric indicators were weak to moderate in strength during this La Niña, including trade winds and cloudiness. The monthly Southern Oscillation Index (SOI) values exceeded the La Niña threshold in 3 out of the 4 months between December 2024 and March 2025. The SOI is point-based and off-equatorial, representing the atmospheric pressure anomaly gradient between Darwin and Tahiti. The equatorial SOI, which represents the broader atmospheric pressure anomaly gradient between the equatorial eastern Pacific Ocean and the equatorial Indonesian region, was consistently typical of a La Niña.

The late event formation along with the influence of other factors (such as the record late start to the monsoon) likely resulted in atypical impacts (compared to La Niña patterns averaged over the historical record) to Australian rainfall. Overall, the weak La Niña did not appear to be the dominant influence, with its timing and strength suggesting that both short- and long-term climate drivers had a greater impact on rainfall outcomes.

Rainfall

For the 6-month period September 2024 to February 2025, rainfall was above average for much of Western Australia and parts of northern Queensland (Figure 1). Rainfall was below average across much of southern Australia.

Spring 2024 rainfall was 36% above the Australian area-average, with western, central and northern Australia receiving above average rainfall (Figure 2). Only scattered areas of eastern Australia, including northern New South Wales and eastern and central Queensland received above average spring rainfall, with much of the eastern states close to average. Spring rainfall was below average across large parts of the southern mainland.

Summer rainfall was above average across parts of the east and north-west, while rainfall continued to be below average over much of southern Australia. Tropical influences such as the late arrival of the monsoon in Darwin dominated Australian rainfall patterns over summer, rather than the influence of a weak La Niña. While the peak magnitude of the relative-Niño3.4 index was observed in January 2025, rainfall was below average across much of northern Australia due to a lack of monsoonal activity (Figure 3).

While the relative Nino3.4 index was back in neutral territory by March, the month saw increased tropical low activity, with rainfall in the top 10% of records across much of north-eastern Australia.

Other impacts

There were 12 tropical cyclones during the 2024–25 tropical cyclone season; slightly above the long-term average of 10 per season, and higher than the recent average of 9 per season since 2000–01.

Historically, during La Niña events, maximum and minimum temperatures were typically cooler than average during summer across much of northern and eastern Australia. However, maximum and minimum temperatures in 2024–25 were warmer than average across most of Australia. It was the warmest spring on record and the second-warmest summer on record since national observations began in 1910.

More recently, while ENSO continues to play a part in year-to-year Australian temperature variability, global warming is the dominant influence on Australian temperatures.

Summary

The 2020–2023 La Niña event saw three peaks in sea surface temperature (SST) anomalies across three years. Signs of a La Niña were apparent during winter 2020, with conditions becoming established in spring and SST anomalies reaching moderate to strong levels by the end of the year. Both the Southern Oscillation Index (SOI) and SSTs returned to neutral values from March to July 2021. The second peak emerged in spring 2021 and persisted through the 2021–2022 summer. Conditions weakened slightly during winter 2022, with the third peak emerging in September 2022, returning to neutral in March 2023.

Rainfall and flooding

Between August 2020 and March 2021, rainfall was above to very much above average (Figure 1) across areas of western, central, and southern Australia and northern Queensland. Conversely, parts of south-eastern Queensland recorded below to very much below average rainfall (in the lowest 10% of all such periods since 1900–1901) during this time. Flooding occurred across far south-eastern Queensland and north-eastern New South Wales during December 2020, with much of the region recording its wettest December on record.

Extreme rainfall and flooding occurred throughout eastern and central Australia in March 2021. The flooding was exacerbated by saturated soils and full catchments.

A dry April to May 2021 was followed by above-average winter and spring rainfall for most of the country excluding Western Australia, with much of eastern Australia recording rainfall totals very much above average (in the highest 10% of all years since 1900) (Figure 2).

November 2021 was Australia's Wettest November on record. In conjunction with La Niña, a negative IOD and positive Southern Annular Mode (SAM) also contributed to the weather patterns responsible for high rainfall between mid-October and November. Highest on record November rainfall totals occurred over large parts of eastern, southern and central Australia, with New South Wales and South Australia recording their wettest November on record. November 2021 was also Australia's coolest November since 1999.

2022 was Australia's ninth-wettest year on record (Figure 3), with several notable wet periods through the year. Extreme multi-day rainfall and flooding impacted south-eastern Queensland and eastern New South Wales from late February to early March. Spring 2022 was the second-wettest spring on record for Australia as a whole (Figure 4), and wettest on record for the Murray–Darling Basin (MDB), New South Wales, and Victoria. It was also the second-wettest cool season for southern Australia behind only 1974. Like spring 2021, a negative IOD and positive SAM also likely contributed to the weather patterns that resulted in the high spring rainfall.

Significant flooding occurred in southern and eastern states during spring 2022. In September, minor to moderate flood levels were reached in Victoria and Tasmania, and several rivers in the northern MDB reached major flood levels. From the start of October widespread major flooding affected New South Wales, Victoria, Queensland and Tasmania. Flood waters impacted both the upper and lower catchments of the MDB, and gradually flowed downstream, reaching South Australia by late December. Western Sydney also saw major flooding at several times during 2022, with Sydney recording its wettest year on record.

The 2020–2023 La Niña event concluded in February 2023 when atmospheric and oceanic indicators returned to neutral conditions. Following this, El Niño conditions begun to form during winter and spring 2023.

Other impacts

Mass bleaching of the Great Barrier Reef occurred during the 2021–2022 summer, the first mass bleaching event during a La Niña.

Tropical cyclone numbers for the Australian region during the 2020–2021, 2021–2022 and 2022–2023 seasons were close to the 21st century average, with 8, 10 and 7 respectively. Historically, La Niña years have generally seen an above-average number of tropical cyclones for the Australian region. However, the State of the Climate 2022 report noted a downward trend in the number of tropical cyclones observed in the Australian region since reliable satellite observations began in 1982.

The 2017–2018 La Niña event was weak to moderate in overall strength and relatively short-lived. La Niña conditions emerged in October 2017, with the relative-Niño3.4 weekly index peaking at −1.36 °C in early December. There was a brief weakening of the La Niña signal in February 2018 with the passage of a strong pulse of the Madden–Julian Oscillation (MJO), but ENSO indices generally returned to La Niña levels until the end of March.

Cloudiness was above average near the Date Line from September 2017 to January 2018, and trade winds were moderately stronger than usual across the central and western equatorial Pacific—consistent with patterns typically associated with La Niña events. The Southern Oscillation Index (SOI) exceeded La Niña thresholds for much of the period when the relative Niño3.4 index was at La Niña levels, with the exceptions of December 2017 and February 2018.

Rainfall

Rainfall for the duration of the event, October 2017 to March 2018, was close to average over the eastern half of Australia, but above average in the west (Figure 1). October to November 2017 saw above-average rainfall for most of the country, particularly for the northern half of Australia (Figure 2). Queensland's October rainfall was the third-highest on record (since 1900) with flooding occurring in coastal regions.. In November and December, much of southern and central Australia, and parts of the tropical north saw above- to very-much-above-average rainfall. In south-east Australia, heavy rainfall in early December resulted in flash flooding and daily rainfall records.

Over the 2017–18 summer, Western Australia had its tenth-wettest summer on record, with parts of the north and south-east of the state seeing highest on record summer totals. However, below average rainfall was observed across much of eastern Australia. Summer rainfall was in the lowest 10% of historical records since 1900 for much of inland and western Queensland.

March 2018 saw above average rainfall across much of northern and western Queensland and the eastern Northern Territory. Parts of northern Queensland observed record-high monthly totals associated with Severe Tropical Cyclone Nora and an active phase of the monsoon. In contrast, below average rainfall was recorded across much of southern Australia, with some areas in the lowest 10% of historical records.

The 2017–2018 La Niña event concluded in late-March 2018 when atmospheric and oceanic indicators returned to neutral conditions. The short-lived and weak nature of this event, in addition to the interaction with strong episodes of the Australian monsoon and Madden–Julian Oscillation, produced an overall weak impact on Australian rainfall patterns.

Other impacts

In November 2017, a long-lasting blocking high over the Tasman Sea caused persistent northerly winds across south-eastern Australia, leading to an unusually warm spell in Tasmania and Victoria (see Special Climate Statement 63). This warmth during spring was driven more by prolonged, low-intensity warm spells rather than extreme heatwaves. This is consistent with past La Niña events, which have historically been associated with extended periods of consistently warm days and nights in south-eastern Australia, rather than spikes in heat extremes.

The impact of this La Niña event on tropical cyclone numbers was limited. The Australian region observed nine tropical cyclones during the 2017–18 tropical cyclone season, compared to the long-term average of 10. Four systems made landfall over north-west Western Australia, contributing to the above average rainfall observed in those regions. March 2018 saw four active tropical cyclones in the Australian region, including Severe Tropical Cyclone Nora.

The 2010–12 La Niña event consisted of two peaks over successive summers; the 2010–11 peak was one of the strongest on record, comparable in strength with the events of 1917–18, 1955–56 and 1975–76. In October and December 2010, and February and March 2011, monthly Southern Oscillation Index values were the highest on record (records commenced in 1876). The 2011–12 peak was weaker, but still of moderate strength, in both atmospheric and oceanic indicators.

2010 and 2011 were the third-wettest and second-wettest calendar years on record for Australia, with 703 mm and 708 mm respectively, both well above the long-term average of 465 mm. Combined, the two events yielded Australia's wettest 24-month period on record (April 2010 to March 2012, Figure 1), and wettest two-calendar-year period (2010–2012), with 1411 mm of rainfall, topping the previous record of 1407 mm in 1973–1974.

The first year of the event followed a typical La Niña pattern, emerging in autumn 2010 and strengthening over winter. A dry June in 2010 was followed by significantly higher than average rainfall over the nine months from July 2010 to March 2011 (Figure 2). The SOI weakened and returned to neutral values during winter 2011. The six months from April to September 2011 (Figure 3) were relatively dry over much of Australia under neutral ENSO conditions. The SOI rose, again in spring 2011 and above-average rainfall returned to most of mainland Australia for the six months from October 2011 to March 2012 (Figure 4), although totals were not as exceptional as during the 2010–11 period.

Flooding was widespread between September 2010 and March 2011. As well as severe flooding in southeast Queensland, large areas of northern and western Victoria, New South Wales, northwestern Western Australia, and eastern Tasmania were subject to significant flooding. Widespread flooding across southeast Australia resulted in a significant increase in mosquito-borne diseases; 1092 cases of Ross River Virus were detected in Victoria between January and April, and 151 cases of Barmah Forest Virus. The tropics also experienced some highly unseasonable rain events during late autumn and winter 2010, in what is typically the dry season. Abundant vegetation growth following high rainfall in the usually arid interior fuelled widespread grassfires in central and northern Australia between August and November 2011. Much of inland southern and far northern Queensland, most of New South Wales, northern Victoria, and central Australia experienced flooding at least once between late November 2011 and March 2012.

Several notable tropical cyclones occurred during the summers of 2010–11 and 2011–12. Severe tropical cyclone Yasi was the most significant – Yasi was the strongest cyclone to make landfall in Queensland since at least 1918 (also a La Niña year), crossing the coast near Mission Beach, between Cairns and Townsville, as a category 5 storm. Other notable cyclones during the two La Niña events included Carlos, Grant, and Lua.

Read more: "Record-breaking La Niña events: An analysis of the La Niña life cycle and the impacts and significance of the 2010–11 and 2011–12 La Niña events in Australia".

This weak La Niña was a very short, marginal event, with its greatest impact occurring across the north of Australia during the period August 2008 to April 2009 (Figure 1).

The La Niña began with close-to-average rainfall recorded over northern Australia during August to October (Figure 2) and below to very-much-below-average rainfall in the southeast. This was followed by a widespread wet November to December (Figure 3), with 62% of Australia recording rainfall in the two wettest deciles. Tropical cyclone Billy produced areas of highest on record falls during December in the region surrounding Wyndham in the NT.

In contrast, January to February 2009 (Figure 4) was very dry across much of southern Australia, with Victoria and SA having 94% and 65% of their area in the lowest decile of rainfall, respectively. This, coupled with two extreme heat waves during the same period, contributed to the Black Saturday bushfires, with 7th February 2009 being the hottest day on record for large parts of Victoria, including Melbourne.

This moderately strong La Niña had a moderate impact over parts of northern and eastern Australia where rainfall was in deciles 8–10 for the nine months from June 2007 to February 2008 (Figure 1). However, it was late in developing and anomalous in terms of the broad band of below-average falls from central to southeastern Australia, due largely to very dry conditions from August to October (Figure 2). As a result there was little significant relief from the long-term dry conditions which had plagued the southeast, especially the high rainfall areas of the Murray Darling Basin. June 2007 (Figure 3) brought decile 10 totals, including large areas with record-high falls, across most of the central and eastern tropics. Wet conditions also extended down the east coast where major floods occurred in the Hunter region of NSW and in Gippsland. More typical La Niña rainfall anomalies took hold from November, with the four months from November 2007 to February 2008 (Figure 4) being moderately wet through the east of the mainland, and the north of both the NT and WA. A few small regions in Queensland and NSW had highest-on-record falls for the four-month period.

Although there was waxing and waning of intensity, the 35 months from May 1998 to March 2001 (Figure 1) can be considered one long La Niña phase. It had a moderate to strong effect on a large area of the country; the south coast and Tasmania being the only areas missing out on the heavy falls. The NT had over 97% of its area in the highest decile of rainfall, while a high percentage of WA received record-high falls. Record-high totals were also prevalent about the northern NT, northeast Queensland and on the northwest slopes and plains of NSW.

The May 1998 to March 1999 (Figure 2) period followed a typical La Niña pattern, both in terms of its timing from one autumn to the next and also with widespread above to very-much-above-average falls. The following six months from April to September 1999 (Figure 3) were considerably drier and seemed to mark a return to neutral conditions. However, the onset of the northern wet season saw a resurgence of La Niña conditions, with widespread heavy rain over much of the country for the eight months from October 1999 to May 2000 (Figure 4). A pattern similar to the previous year then recurred with a dry spell from June to September 2000 (Figure 5) prior to a wet six months from October 2000 to March 2001 (Figure 6). With an Australia-wide average of 698 mm, the calendar year 2000 (Figure 7) was the second wettest year since 1900; only 1974 (760 mm) was wetter.

Flooding was rife during the three-year period, with tropical cyclones such as Thelma in December 1998, Rona in February 1999, Vance in March 1999, Steve in February and March 2000, Sam in December 2000 and Abigail in February 2001 all causing significant flooding in their wake. Substantial crop and stock losses were caused by widespread flooding in NSW and Queensland during July to September 1998 (Figure 8). Some regions had as many as four separate flooding incidents during these three months. There were also numerous hailstorms during this La Niña, including the notorious Sydney hailstorm in April 1999, which is one of Australia's most costly natural disasters in monetary terms.

This was a strong La Niña which had a strong effect on the southeast half of the country, with April 1988 to July 1989 (Figure 1) rainfall in the highest decile for most New South Wales, much of South Australia and northern Victoria, and parts of southern and eastern Queensland. Much of Tasmania and western WA, together with parts of the far north also had falls in deciles 8 and 9 for this period. From April to December 1988 (Figure 2), above to very-much-above-average rainfall fell across much of the country, but there was a marked break to this pattern over January and February 1989 (Figure 3), when much of the continent recorded totals in the lowest three deciles. However, typical La Niña conditions returned with a vengeance during the March to July 1989 (Figure 4) period when there were widespread falls in decile 10, including record-high totals over a large fraction of SA.

Flooding was a regular occurrence during this La Niña, with flooding in Adelaide in May 1988, NSW, northern SA and western Queensland during July 1988, southeastern Queensland in September 1988, SA in March 1989 and central Queensland during April to May 1989. One of the more notable floods occurred in Victoria during November and December 1988 (Figure 5), with widespread flash flooding, particularly in the Melbourne and Gippsland areas. Several rivers broke their banks.

In December 1988 following the flooding, over 700 cases of Ross River virus were reported in Victoria, with the main affected area being the Gippsland region. In April 1989, following heavy rains in northeast Tasmania almost 60 cases of Ross River virus were reported.

Following a relatively intense El Niño, and in perhaps the longest sustained period of La Niña conditions in the instrumental record, this strong La Niña had a strong effect on Australia, with excessive rainfall over much of the country. For the period from June 1973 until March 1976 (Figure 1), rainfall was above average over virtually the entire country, with more than half of the area east of WA recording its highest rainfall on record for this particular 34-month period. The particularly impressive feature of the rainfall during this period was the complete lack of significant dry periods; only two short periods are worth noting – June to July 1974 and May to June 1975. With an area-average of 760 mm, 1974 was Australia's wettest year on record, while 1973 (651 mm) and 1975 (602 mm) were third and fifth wettest respectively.

The very wet 1974 (Figure 2) caused the most recent major outbreak of Murray Valley encephalitis in southeastern Australia, with 58 reported cases. Also during this La Niña, 400–500 cases of Ross River virus were reported, ranging in areas from South Australia's Murray Valley region, to Queensland. Most of these cases occurred in SA. The extensive rain in early 1974 produced abundant growth in central Australia, and when it dried out in late spring 1974 it provided abundant fuel for widespread fires.

In what was a remarkable period, the month of January 1974 (Figure 3) stands out, with record rainfall and widespread flooding in central and eastern Australia. In area-average terms it is the wettest month on record across Australia, the NT and Queensland, while it is the second wettest over SA and NSW. Many of the major river systems in these areas were flooded during this time. Towards the end of January, tropical cyclone Wanda crossed the coast near Brisbane and exacerbated the flooding in Queensland, including one of Brisbane's worst floods on record. Several other cyclones caused flooding, such as Una in December 1973, Zoe in March 1974, David in January 1976, Beth and ex-tropical cyclone Alan in February 1976, and Dawn in March 1976 to name a few.

Another standout month was October 1975 (Figure 4) during which Victoria had its wettest month on record and extensive flooding, and a large area with highest-on-record falls. Most other parts of continental Australia had rainfall in the top decile, except for southwest WA, northern NSW, southern Queensland and scattered areas in the NT.

This moderately strong La Niña had only a weak to modest effect on Australia's climate. Over the 22-month period from June 1970 until March 1972 (Figure 1), rainfall was above average over much of the area comprised of Queensland, NSW, Victoria, Tasmania and the southeast of SA. Tasmania was particularly wet, with most of the state having totals in the highest 10% of the historical record (decile 10), as did southwest Victoria, coastal SA and some parts of eastern Queensland.

Overall though, the rainfall was fairly patchy in terms of sustained high falls, while some notable dry periods occurred during the event. Apart from in Tasmania and southern Victoria, winter 1970 (Figure 2) was especially dry, with decile 1 totals in a broad zone extending across the centre of the country. Summer 1970–71 was drier than average over the NT, western Queensland, SA and eastern WA, while the March–June period in 1971 was much drier than average from southeast Queensland, through eastern NSW to East Gippsland.

There were several floods during this La Niña. In August 1970, 89% of Tasmania had monthly rainfall in decile 10, with flooding on the Deloraine and La Trobe rivers in the north of the state. Flash floods occurred in Canberra in January 1971, while in February of that same year, there was widespread flooding in the Gippsland area of Victoria, which caused large agricultural losses. March brought flooding in Broken Hill.

Several cyclones also contributed to flooding, notably tropical cyclone Dora in February 1971, which hit the Queensland coast just north of Brisbane. The affected region had highest-on-record falls for February. Another tropical cyclone, Althea, crossed the coast at Townsville on Christmas Eve 1971, causing $50 million damage (1971 dollars). Although it didn't cause particularly large amounts of flooding, the cyclone generated a 3.66 m storm surge and damaged or destroyed 90% of the houses on Magnetic Island. In the decile map for December (Figure 3), Althea’s path is quite clear, with the arc of the highest decile in Queensland showing its path.

There were also two outbreaks of Ross River Virus during this La Niña: one in December 1970 in Coleambally, NSW, with 33 reported cases, and another one in February 1971 in the Murray River region, with over 109 cases reported.

By February and March 1972, regions of below-average rainfall were becoming evident as the climate began a shift into El Niño.

This short-lived La Niña had a modest impact on parts of southern and eastern Australia. As the El Niño of 1963–64 came to an end, the SOI began rising during autumn, with widespread above-average falls during April, especially across the southern half of the country. However, May was very dry in southwest WA, while elsewhere in the south falls were rather patchy.

The June to October period was marked by above-average falls across the southern one-third of Australia, particularly about the exposed coast and ranges in the southeast. Record-high July totals were recorded near parts of WA's southern coast, in a large straddling southeast SA and western Victoria, and across Victoria’s northeast ranges into the Snowy Mountains of NSW. Above-average falls were widespread over Australia in September and October, especially the former when record-high totals were observed over much of central to southeastern SA and the adjacent parts of northwest Victoria and the far west of NSW.

For the seven months from April to October 1964 (Figure 1), rainfall was in deciles 8–10 over southern WA, western and northern Tasmania, Victoria, much of the southern two-thirds of SA, a majority of NSW and parts of both eastern Queensland and the north of the NT. In a few small patches in western Victoria it was the wettest such period on record.

Wet conditions persisted in the far southeast during November and December, but below-average falls prevailed over NSW and the southern half of Queensland as a prelude to the 1965 El Niño.

This La Niña had a strong effect across the eastern third of Australia. From April 1954 until January 1957 (34 months) (Figure 1), Queensland, NSW and Victoria all had over 70% of their areas in the highest decile of rainfall, with parts of these areas being highest-on-record falls.

In February 1955 (Figure 2) extensive flooding occurred in NSW, with almost all of the state's river systems affected. As a result, 15 000 people were left homeless in the short term, 50 people died, 100 000 livestock died and there were large agricultural losses. Some of the worst flooding occurred at Branxton, just north of Newcastle, where flood waters were almost 4m deep, and towns such as Maitland were also largely inundated.

March 1956 to December 1956 (Figure 3) was a very wet period for the regions surrounding the Murray-Darling River System, with floods from May until December. 1956 was the wettest year on record for the Murray-Darling Basin. During this time there was a Ross River virus outbreak in April 1956, with over 2000 cases reported throughout South Australia, Victoria and NSW near the river system.

A particularly wet La Niña, which probably classifies alongside 1916–1918 and 1973–1976 as those with the strongest effects. The oceanic signal of this event began in winter 1949 when cooling occurred along the equatorial Pacific, but it wasn't until December 1949 that a consistent SOI signal became established. For the time period from December 1949 until February 1951 (Figure 1), much of eastern and northern Australia recorded above to very-much-above-average rainfall. Notably, Queensland had 95% of its area in the top two deciles for this period, and NSW 76%, with significant parts being highest on record, especially over the eastern half of NSW. 1950 was the wettest year on record averaged over both Queensland and NSW as well as over the four eastern states that comprise eastern Australia, despite the fact that Tasmania had its eighth driest year on record.

Several cyclones hit the north-east coast in early 1950, with one in particular during March (Figure 2) causing widespread flooding. Winter 1950 (Figure 3) was especially wet in coastal NSW and southern Queensland, while spring 1950 (Figure 4) was wet across much of the region, with record falls across large parts of Queensland and NSW west of the Great Dividing Range.

The wet year of 1950 (Figure 5) also caused Murray Valley encephalitis to reappear in 1951, with 48 reported cases.

For the first part of this La Niña, its effects were rather widespread from May to December 1942 (Figure 1), with most of the continent getting decent falls, and almost half of the area being in the top two deciles. The second part, however, was quite dry for eastern Australia, which had approximately half of its area in the bottom 30% of rainfall for January to April 1943 (Figure 2). Western Australia by comparison, fared better in this period, with above-average falls across most of the coast, and southwest WA. These above-average falls in WA were caused by several tropical cyclones making landfall during this period.

In the region around Darwin in the Northern Territory, 105 cases of Ross River virus were reported in November 1942. In the following November another 40 cases were reported in the Adelaide River area nearby.

A dry La Niña, especially so in the southeast. April 1938 to January 1939 (Figure 1) was very dry in Victoria, southern NSW and southeastern South Australia. Victoria had 98% of its area in the lowest decile, and southeast Victoria had lowest-on-record falls. When this low rainfall combined with extreme heat, it resulted in the Black Friday fires, which lit up almost all of Victoria, southern NSW and southeastern South Australia, causing massive devastation. Some 1.5–2 million ha was burnt in Victoria, much of it protected forest.

In the three months following Black Friday, February–April 1939 (Figure 2) these regions received rainfall in the top decile, some parts receiving record-high falls.

Another anomalously dry La Niña in Australia, with Tasmania and northern Australia being the only substantial regions to get above-average rainfall from August 1928–March 1930 (Figure 1). In South Australia it was the driest such period on record. In northern Tasmania flooding occurred in April 1929 (Figure 2). 14 people died when the Briscia Dam (on the Cascade River) burst; to date Australia's only failure of a major dam that has caused casualties. Flooding and livestock losses occurred as far south as Hobart.

Several cyclones also caused flooding in north-east Australia during this La Niña. Two hit the east coast within a week of each other in February 1929, causing widespread flooding. Another pair of closely spaced cyclones passed though again in January 1930 in northern Queensland. The first of this second pair of cyclones caused devastating flooding in most of the rivers between Cooktown and Townsville, while the second, late in the month brought heavy rains in which 10 000 livestock (cattle and sheep) drowned.

The initial three months of this La Niña, May to July 1924 (Figure 1) were rather dry, and the higher-than-average rainfall fell mainly in late winter and springtime. For August 1924 to January 1925 (Figure 2) much of eastern Australia, the NT and also southwest Western Australia had rainfall totals in the top 10%. A small section of central Queensland had highest-on-record falls. In May 1925, an east coast low caused exceptionally heavy but fairly localised rainfall, which resulted in flooding on the Murrumbidgee River and the coastal rivers of southern NSW, affecting southern NSW and the ACT, with Gundagai and Wagga Wagga severely affected.

The overall impact for this La Niña may be described as weak, with only slightly higher than normal rainfall for May 1924–May 1925 (Figure 3).

Following the drought of 1911–1916, this long lasting La Niña produced highest-on-record falls for the 20-month period from June 1916 to January 1918 (Figure 1) in regions of SA, Victoria, Tasmania, WA, NSW and a large area of Queensland. All the eastern states and SA had well over half of their area with rainfall in the highest 10% of the historical record during this time.

Queensland was hardest hit by this La Niña. Heavy rain during December 1916 caused flooding in Clermont in which most of the town was destroyed or swept away. Following the flood the town was rebuilt on higher ground.

In addition, two intense tropical cyclones affected Queensland in early 1918. Now known as the Mackay cyclone, this storm drenched the central Queensland coastal town with 1411 mm of rainfall in three days. In March an even stronger cyclone made landfall over Innisfail, with widespread damage affecting the Cairns, Babinda and Atherton Tableland districts.

Heavy rain across Victoria in September 1916 not only caused extensive flooding, but aided in producing high wheat yields for the Mallee and Wimmera regions. In June 1917, there were floods in northeastern Victoria.

During the very wet year of 1917 (Figure 2), 114 cases of Murray Valley Encephalitis were reported in southeastern Australia, and another 67 cases in 1918.

This La Niña had a rather localised yet strong effect, with high falls in coastal Queensland, southeast SA, western Victoria and south west WA during May 1909 to April 1911 (Figure 1). The rest of the continent received mostly normal falls for this period, except for central Western Australia, which had well below-average rainfall. During this time floods were frequent, with several events in the summer months in Queensland.

During August 1909 (Figure 2) northern and western Victoria, and parts of SA, had a significant flooding event, affecting several rivers. Winter 1909 was the third-wettest on record for Victoria. The summer of 1910–11 was also particularly wet through most of eastern Australia. From early to mid-1911, the continent began to dry, marking the beginning of the 1911–1916 drought, which affected most of Australia.

This La Niña had a moderate but relatively short-lived effect, with above-average rainfalls largely confined to the period August 1906 to January 1907. The three months August–October 1906 (Figure 1) were especially wet, with large areas recording decile 10 totals including record-high falls through western parts of Queensland and NSW, as well as over the north of the NT and northwest WA (although in the latter case totals were modest). Average to above-average falls continued for another three months such that the six months from August 1906 to January 1907 (Figure 2) saw above to very-much-above-average totals over most of eastern and northern Australia. February 1907 effectively marked the end of the event. A neutral SOI pattern became established, with the period from February to April 1907 (Figure 3) being rather dry by comparison: large areas of eastern Australia had falls in deciles 2 or 3 while above-average totals were confined to small patches.

This La Niña followed the Federation drought (1895–1902). Over the 12 months from March 1903–February 1904 (Figure 1) this episode had a moderate to strong effect, with above-average falls over southern and central Australia, Victoria and coastal Queensland. South Australia and the Northern Territory both had over 95% of their areas in the top 3 rain deciles. November 1903–January 1904 (Figure 2) was particularly wet in the north, with parts of the Northern Territory having highest-on-record falls. The return to drier conditions occurred in winter 1904 (Figure 3) with parts of the far north eastern coast of Queensland having their driest winter on record.