South Australia, Victoria, Tasmania, the ACT and NSW are set to swelter through oppressive temperatures into the weekend as a heatwave grips southeastern states, the Bureau of Meteorology said.
The conditions are set to be the worst since the 2019-20 summer bushfires that killed more than 30 people and burned through millions of hectares.
"The burst of high temperatures starting from today will be the beginning of a major heatwave across southeast Australia," meteorologist Helen Reid said on Wednesday.
SA and Victoria will be the first in line to experience the sweltering conditions on Wednesday.
Temperatures across all of SA are set to reach the low- to mid-40s, with Adelaide set for a maximum of 42C and some inland and southeastern parts pushing 47C.
"A heatwave is not just a normal spike of summer heat," Ms Reid said.
"Extreme heatwaves are likely to affect all Australians to some degree, not just those who are vulnerable, such as the elderly or those who are unwell, but healthy and active people as well."
Victorians will steam in temperatures reaching the low- to mid-40s, with Melbourne reaching 42C and inland parts through to the north reaching 46C, as severe heatwaves build and extend statewide.
Total fire bans in place for the Wimmera, South West and Central regions, including Melbourne and Geelong for the first time this summer.
Victoria's Emergency Management Commissioner Tim Wiebusch is warning of challenging extreme heatwave conditions "not seen since 2019-20".
He said there is a very real risk of serious fires starting off the back of underlying dry conditions.
Country Fire Authority chief executive Jason Heffernan warned there aren't many parts of the state immune to the fire conditions over the next 72 hours, after good rainfall led to prolific grass growth, adding to ground fuel.
It will also be a warm day for Tasmania with low to severe heatwaves building in the state's northeast.
The heat is expected to move into the ACT and NSW heading into the weekend, with the capital expected to peak at 42C on Friday while Sydney reaches 39C on the final heatwave day on Saturday.
People are being urged to check in on vulnerable young and older people and those who are unwell, never leave children or pets alone in the car, keep cool and drink plenty of water and shelter in cool places during the hottest part of the day.
The number of call-outs involving family members locked in cars has increased 26 per cent year-on-year, according to SA's Royal Automobile Association.
Patrols attended 370 call-outs to rescue children from cars and 423 call-outs to free pets in 2025.
The Royal Australian College of GPs has also issued a warning urging the public to prioritise hydration, rest indoors, and check on people more susceptible to heat exhaustion and heat stroke.
"Heat waves can be deadly, and the Bureau of Meteorology has warned that this promises to be the 'most significant' heatwave south-eastern Australia has seen in six years," President Dr Michael Wright added.
Australia experienced its fourth-warmest year on record in 2025, while 2024 was the nation's second-warmest year.
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