Parts of South Australia have been declared a catastrophic fire risk and Victorians have been warned of 'blast furnace" conditions as a heat wave punishes Australia's south eastern states.
Total fire bans have been declared throughout SA and Victoria and across most of Tasmania for Saturday.
A catastrophic fire risk has been declared for SA's Lower Southeast, Upper Southeast, and extreme ratings have been issued for the Mid North and the York Peninsula.
Extreme ratings have been issued in Victoria's Mallee, Wimmera, South West and Central districts.
Adelaide will have a top temperature of 41C while the mercury is forecast to reach the mid-40s in some regional and outback towns.
A strong cool change is also expected to sweep across SA during the day, reaching Adelaide mid-afternoon, with the changing conditions adding to the fire risk.
CFS chief officer Greg Nettleton said the combination of high temperatures, strong winds and low humidity was a "recipe for fire".
"High temperatures, low humidity, strong winds, fuel, it's all there," he said.
Meanwhile Melbourne is set to swelter through its hottest day in two years on Saturday, forecast to reach 42C, and the mercury could even hit 45C in Warracknabeal in the northwest.
"This heat is a killer. It's going to be like a blast furnace tomorrow and you need to adjust what you do," Ambulance Victoria's state health commander Paul Holman told reporters on Friday.
Temperatures are forecast to reach the mid-30s in Tasmania's east and south with wind gusts of up to 45km/h, the Bureau of Meteorology predicts.
A total fire ban will be in place for King Island and northern and southern Tasmania for all of Saturday.
More than 200 fire brigades are on standby across the state, with specialist response teams and aircraft also ready to go.
In NSW, temperatures across the state will be above 40C, and could reach 45C at Menindee in the far west and Ivanhoe in the state's centre.
People are being warned to keep safe if heading to the beach after several drownings since December.
"Lifesavers and lifeguards are out in force but we need the public to take some responsibility for their own safety," Surf Live Saving NSW chief executive Steven Pearce said.
"The tragic incidents we have witnessed through December and early January are a reminder of the power of the ocean."