Far-reaching heatwave conditions are on Thursday impacting central Western Australia, northern South Australia, the Northern Territory, southern Queensland and NSW.
Bureau of Meteorology warnings, some severe, are in place for all five states, while locally extreme heatwaves could impact parts of far southwest Queensland.
"Widespread temperatures above 40C are expected across inland and eastern parts of Australia over the next few days, with some inland areas as hot as 49C," a spokesperson told AAP.
"The heat will spread out to the coast, with Sydney forecast to reach 35C on both Thursday and Friday, while some of the western suburbs may reach 39C or even 40C."
Despite cloudy conditions across Sydney on Thursday, the city's airport observation tower had already reached 29C by 10am, while Gosford on the NSW Central Coast had tipped 33.3C.
Temperatures in the high 30s are set to sweep the NSW coast with Newcastle forecast for 39C on Thursday.
Inland areas of Southern Australia's north, southwest Queensland and northwest NSW could swelter in temperatures in the mid to high 40s on both Thursday and Friday.
Roxby Downs, about 500km north of Adelaide, was expected to reach 44C on Thursday, while further inland the outback town of Coober Pedy was forecast to tip 42C.
Queensland's Birdsville could record a peak of 49C on Friday.
Although widespread records were not expected to be broken, a few remote or inland locations could possibly set new records for maximum temperatures, the bureau said.
Cooler air that was developing in the south of Western Australia on Wednesday is set to gradually spread across the southern states on Thursday and Friday, ending the heatwave for many.
The forecast change will bring cooler weather for almost all areas on the weekend.
