Penrith and Richmond in Sydney's western suburbs are forecast to reach 44C on Saturday, with 40C expected in the city.
Inland areas will also be hot and windy, with the nation's capital set to reach 37C.
Health authorities have urged people to check in on elderly friends and neighbours. (Nikki Short/AAP PHOTOS)
Health authorities have issued heatwave alerts urging people to avoid the outdoors during the hottest part of the day, to check in on elderly friends and neighbours and to keep pets cool.
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.
In SA, temperatures at Coober Pedy and Leigh Creek topped 43C with the mercury at Woomera Aerodrome climbing to 46.4C.
A cool change is set to bring relief to the southern states with a maximum of 17C forecast in Adelaide and 19C in Melbourne.
Warnings are still in place further north for Saturday, with the NSW Rural Fire Service issuing total fire bans for five state districts including greater Sydney.
Saturday is expected to be the hottest day of the NSW heatwave which is notable for its spread across the state.
"That intensity is really coming across the Dividing Ranges, bringing those severe heatwave conditions with them," the Bureau of Meteorology's Mariam Bradbury told AAP.
She said a sea-breeze will not save coastal areas.
"This is not just the Sydney area. All the way up to the Newcastle coast, temperatures are pushing 40 degrees," she said.
Ms Bradbury said the cool change will continue to push up the east coast later in the weekend, but inland areas won't see much of a reprieve from the high temperatures.
"Sydney is going to see a bit of relief going into Sunday with the cool change, but it's going struggle to penetrate inland," she said.
A top of 27C is forecast for Sydney on Sunday.
27°C