South Australia, Victoria, Tasmania, the ACT and NSW are set to swelter through oppressive temperatures into the weekend, the Bureau of Meteorology says.
The conditions may 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 are first in line to experience the sweltering conditions on Wednesday.
Victorians are steaming in temperatures reaching the low- to mid-40s, with Melbourne expected to reach 42C and inland parts through to the north reaching 46C.
An emergency-level fire is burning out of control on the NSW/Victoria border at Bungil, Granya and Thologolong east of Albury/Wodonga.
Residents are being told to leave immediately before conditions become too dangerous.
Another fire at Moyreisk, Natte Yallock and Redbank in the state's central west has been downgraded from emergency level to watch and act after firefighters were able to slow the spread, but people are being warned the situation remains dangerous.
Total fire bans are 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 warned of challenging extreme heatwave conditions "not seen since 2019-20", with a very real risk of serious fires starting off the back of underlying dry conditions.
Country Fire Authority chief executive Jason Heffernan warned there weren'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.
Temperatures across all of SA are set to reach the low- to mid-40s, with Adelaide expecting a maximum of 42C and some inland and southeastern parts pushing 47C.
"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," Ms Reid said.
It is also 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 national capital expected to peak at 42C on Friday, while Sydney reaches 39C on the final heatwave day on Saturday.
The Royal Australian College of GPs is urging the public to prioritise hydration, rest indoors and check on people more susceptible to heat exhaustion and heat stroke.
People are also being urged to never leave children or pets alone in the car, with the number of call-outs involving family members locked in cars increasing 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.
Australia experienced its fourth-warmest year on record in 2025, while 2024 was the nation's second-warmest year.
40°C