What are flood classifications?
Flood classifications describe possible impacts when a river reaches certain levels at a specific location. That is, where the water level is measured at a gauge in the river.
At many river locations in Australia, flood levels are classified as minor, moderate and major.
What is minor flooding?
Minor flooding generally causes inconvenience. It may impact low-lying areas near water courses. Roads, low-level bridges, bicycle and pedestrian paths may be under water.
Minor flooding may affect some gardens and buildings below the floor level. In rural areas, livestock and equipment may need to be relocated.
What is moderate flooding?
Moderate flooding may cause more significant impacts.
Main traffic routes may be affected. Some buildings may be affected above floor level. Evacuation may be required. In rural areas, removal of livestock is necessary.
What is major flooding?
Major flooding may cause large disruption and community impacts.
At the major flood level, large areas are generally under water. Many buildings may be affected above floor level. Properties and towns are likely to be isolated and major rail and traffic routes impacted. Evacuation may be required. Utility services such as power may be affected.
What is a flood extent?
Every flood is different. How far a flood will spread is known as the flood extent.
State emergency services, local government and water catchment authorities are responsible for providing information about flood extent.
Video: Understanding flood classifications
Riverine floods typically occur 6 or more hours after heavy rainfall. In Australia, we have 3 riverine flood classifications: minor, moderate and major.
Minor flooding refers to inundation of low-lying areas next to watercourses. Minor roads may be closed and low-level bridges submerged. In urban areas inundation may affect some backyards and buildings below the floor level as well as bicycle and pedestrian paths.
For moderate flooding the area of inundation is more substantial. Main traffic routes may be covered and some buildings may be affected above floor level. Evacuation of flood affected areas and removal of stock may be required.
Finally, major flooding can be very dangerous and may result in extensive inundation of rural and urban areas. Properties and towns are likely to be isolated and major traffic routes and rail routes are likely to be closed. Evacuation of people from flood affected areas may be required and utility services may be impacted.
The Bureau produces warnings and advice to local communities who are experiencing, or are at risk of experiencing riverine flooding.
Flood Watches provide early advice of a developing situation that may lead to flooding. While Flood Warnings are issued when floods are likely to occur or are already occurring.
Sometimes short bursts of heavy rainfall can lead to rapid onset flooding, known as flash flooding. When this is likely to occur, the Bureau will issue a severe weather warning for heavy rain that may lead to flash flooding.
Current flood watches and warnings, as well as severe weather warnings can be found on our warnings page or the BOM weather app. You can also access rainfall and river level data for around 3,500 locations across Australia.
Sadly, fatalities are common around flood waters. Just 15 cm of water is enough to make your car float. So remember, the best way for you to stay safe is to stay out of flood water and follow advice from local emergency services.
How river levels are classified
Organisations that are part of state flood warning consultative committees decide the flood classification levels based on river heights for their local areas. These organisations include:
- state and territory government agencies
- local councils
- emergency services
- catchment management authorities
- water authorities.
They work with us to develop, update and regularly review these classifications and service levels. Community feedback also helps inform this work.
Riverine flood levels for each state and territory are set out in service level specifications for floods.
How are river heights measured?
River heights are usually measured in metres at a specific location. Some rivers are measured against the gauge, while others are measured against sea level.
Gauge measurements
In most cases, river height is relative to the starting point for measurements on the gauge. At some sites, 0 metres means there is no river flow.
Sea level measurements
In many tidal areas and a few inland areas, river levels are expressed in metres above sea level. This is the Australian height datum (AHD).
Observed river heights from the gauge in the Isaac River at Yatton show it peaked above the major flood level on 13–15 January 2026
Observed river height
The water level measured at a gauge in the river is the observed river height. Our river conditions information uses observed river heights. Water levels will vary across the floodplain.
Our flood warnings may include an observed peak river height at some locations. The peak river height is the highest river level observed at a location during a flood.
At many locations, river levels may peak within a few hours or days after heavy rainfall. In larger rivers, peak levels may not occur until weeks or months after the rain has fallen.
Predicted river height
Predicted river height is the water level we forecast at locations along the river.
Our warnings may include when a river is likely to exceed minor, moderate or major flood levels. Forecasts may also be provided for the peak river height and when it is likely to occur at some locations.
During floods, we update the forecast river heights as more information becomes available or the flood risk changes. We include this detail in our flood warnings.
We measure the accuracy of flood forecasts and warnings.