Scorching temperatures and an extreme bushfire risk are in store for Adelaide and much of South Australia today.
The city could post a temperature record, with a 46 degrees Celsius peak forecast, close to 46.1C in 1939. At 8am today (10am NZ time) the city was already at 34.9C.
Adelaide was expected to have a top of 40C on Friday, before a cool change sweeps across South Australia, ending a five-day heatwave, the third worst in the state's history.
Melbourne was expected to hit 44C, while Sydney, saved from the worst of the heat, was only expected to hit 24.4C.
Health warnings have been issued and emergency services are bracing for fire danger as the heatwave sweeps across the country.
The Country Fire Service (CFS) has been fighting a string of fires in the Adelaide Hills, the mid-north, the southeast and across Eyre Peninsula.
An extra five water-bombing aircraft will arrive in the state today to bolster local resources.
The State Emergency Service (SES) urged people to continue to be vigilant to protect themselves and others as best they can from the extreme conditions.
SES spokesman Scott Turner said people should "look after each other, look after the community and survive until we see some reprieve".
Officials in the state of Victoria were preparing for the most extreme weather since the Black Saturday bushfires of 2009, which killed 173 people and destroyed thousands of homes. The heatwave that came before those fires was responsible for 374 deaths.
At the Australian Open in Melbourne, temperatures topped 42C on Tuesday .
Tennis players and ball boys suffered from heat-stroke, China's Peng Shai vomited on court in the heat, and Canadian player Frank Dancevic hallucinated a cartoon character before collapsing.