Australia's climate has warmed by a little over 1 °C, with eight of Australia’s top ten warmest years on record occurring since 2005. Observations show warming in both maximum and minimum temperatures and at all timescales. This warming has seen a significant increase in the annual frequency of daily extreme heat events. The chart showing the increase in extreme daily heat is consistently one of the most reproduced and requested figures from the Bureau-CSIRO State of the Climate report. One reason for this may be that it shows the effects of the warming trend are occurring now in a metric that is highly impactful.

The Bureau of Meteorology's Australian Climate Observations Reference Network – Surface Air Temperature Version 2 dataset is examined for trends in the frequency of daily extreme heat events. Extreme days are those above the 99th percentile of each month from the years 1910–2017. These extreme daily events typically occur over a large area, with generally more than 40 per cent of Australia experiencing daily temperatures in the warmest 10 per cent for that month on the days concerned. Increases in the frequency of extreme events are seen on an annual scale with a seasonal breakdown of these numbers showing an increased frequency of extreme heat across all seasons.

This increase in extreme heat observed across all seasons is an important result for planning purposes. More frequent summer heatwaves, which can adversely affect human health, will need to be planned for. Furthermore, an increased frequency of relative extreme heat in seasons such as autumn and spring can have large impacts on, for example, agriculture production and bushfire weather during these periods.