Adelaide has recorded its hottest ever run of December days, with the mercury hitting 40 degrees Celsius on Saturday.
The Bureau of Meteorology (BoM) confirmed that Adelaide was sweltering through its fourth consecutive day over 40C, making it the city's longest December heatwave since records began.
BoM senior forecaster Mark Anolak the South Australian capital would likely reach 44C this afternoon.
"We have a high pressure system over the Tasman Sea that's not really moving anywhere, we've got anti-clockwise winds around that high pressure system and very hot and dry northerly winds bringing very hot air from inland Australia's towards Adelaide," Anolak told The Huffington Post Australia.
He said many other parts of South Australia would probably hit the mid-40s on Saturday.
A cool change was expected to sweep across the state on Saturday night or Sunday morning, the forecaster added.
It is the second outbreak of extreme weather in Adelaide this month and December 7 was Adelaide's hottest December night ever, with a minimum of 30.3C.
On Friday, the heatwave was responsible for landing an apprentice construction worker in hospital in a critical condition.
He was one of many South Australians who were treated for heatstroke due to the unusually hot conditions.
Meanwhile, Melbourne is also looking at its third very hot day in a row, with the temperature set to reach 35C on Saturday.
BoM forecaster Claire Yeo predicted a cool change to arrive in the Victorian capital on Saturday night.
However, she warned of increased fire danger as the cooler conditions took hold.
"The problem with these kind of conditions, particularly of concern to us from a fire weather perspective is as this change arrives, we are potentially going to see dry thunderstorms developing ahead of the change," she told the ABC.
In addition to Victoria, fire weather warnings are current for Western Australia, South Australia and Tasmania.