Drought in general means acute water shortage. The drought maps highlight areas considered to be suffering from a serious or severe rainfall deficiency. These classes are assigned by first examining rainfall periods of three months or more for selected places throughout Australia to see whether they lie below the 10th percentile (lowest 10% of records). The terms serious and severe are defined by:-

• Serious rainfall deficiency:- rainfall lies above the lowest five per cent of recorded rainfall but below the lowest ten per cent (decile 1 value) for the period in question,
• Severe rainfall deficiency:- rainfall is among the lowest five per cent for the period in question.

Areas where the rainfall is lowest on record for the given time period are also shown. Drought declaration is the responsibility of State Governments which must consider other factors apart from rainfall, however the Bureau of Meteorology's Drought Watch Service assists by providing rainfall information to all interested parties.

Defining the end of a period of rainfall deficiency is a difficult matter, and presents more problems than defining the start. However, the following definitions which were used in the days of hand-drawn analyses, still provide useful guidance as to whether there has been significant relief from a period of rainfall deficit. They are only applicable to periods of about 12 months or less, and great care in their interpretation is needed in those areas which have a distinct wet and dry season in the annual rainfall cycle. The criteria are:

• The rainfall of the past month already exceeds the 30th percentile for the three-month period commencing that month, or
• Rainfall for the past three months is above the 70th percentile for that period.

For other definitions, see also deciles and percentiles.