By Maeve Bannister, Kathryn Magann and Luke Costin in Sydney

Tens of thousands of Australians have sweltered through extreme temperatures as authorities brace for more dangerous conditions.

Heatwave warnings were in place on Friday across parts of all mainland states, prompting extreme to catastrophic fire danger alerts to be issued for sections of South Australia, northern Victoria and outback NSW.

Temperatures in much of the nation’s centre topped 40C, while parts of Sydney, Melbourne, Adelaide and Canberra all experienced maximums in the mid-30s or higher.

Even warmer conditions are expected in the east on Saturday.

In South Australia, temperatures at Coober Pedy and Leigh Creek topped 43C with the mercury at Woomera Aerodrome climbing to 46.4C.

Marree Aero was the highest temperature recorded by the weather bureau on Friday, climbing to 46.5C.

The temperature at Richmond, on Sydney’s northwest outskirts, topped 40C, ahead of forecast maximums in the mid-40s across the city’s west on Saturday.

Smithville in the NSW upper west hit 44.4C on Friday, with Fowlers Gap reaching 42.6C.

Miriam Bradbury from the Bureau of Meteorology said the heat was widespread across much of the country, with the eastern seaboard most impacted.

“The most intense heat is across inland NSW and most of South Australia,” she told AAP.

“But these really intensely hot temperatures do extend elsewhere.

“Northwestern Victoria, southwest Queensland and large parts of the NT and northern Western Australia.

“So, essentially, a huge area of heat extending across the country.

“It’s the latest hot day in a string of really, really hot days.”

A cool change was due to reach southern parts of SA by late on Friday, dropping Adelaide’s overnight temperature down to 15C.

The cool change will then push into southwest parts of Victoria overnight.

The SA Country Fire Service warned Friday would be “one of the most significant fire weather days in several years”.

Nearly 25,000 customers were without power in the state on Friday afternoon, the majority of them in areas north of Adelaide, as the extreme weather conditions wreaked havoc on electricity networks.

Winds were expected to ease through Saturday, but thunderstorm warnings were in place for southern parts of SA.

A total fire ban was in place for several NSW areas before even warmer conditions on Saturday, when the bans will continue.

Ms Bradbury said the extreme heat would push right along the NSW coast after earlier being confined to inland areas.

Almost 20 NSW government schools were closed on Friday due to bushfire risk.

High temperatures sparked a warning to employers to protect their workers and develop a workplace heat-management plan to ensure safety.

“We don’t want to see anyone needing medical attention due to heat exposure,” SafeWork NSW head Trent Curtin said.

Parts of Queensland, Western Australia, the Northern Territory and ACT have also copped soaring temperatures that are expected to intensify over the weekend.