Penrith and Richmond in Sydney's outer west are expected to endure a top of 44C on Saturday, with 40C forecast in the city.
Inland will also be hot and windy, with the ACT headed for a maximum of 37C.
Health authorities have issued alerts for people to avoid the outdoors during the hottest part of the day, check on the elderly and keep pets cool.
Inner Sydney is expected to reach 40 degrees on Saturday. (Nikki Short/AAP PHOTOS)
Heatwave warnings are in place across parts of all mainland states, with high to catastrophic fire danger alerts issued for sections of South Australia, northern Victoria, outback NSW and Greater Sydney.
In SA, about 35,000 households lost power on Friday due to catastrophic bushfire risk, dry lightning and strong winds.
SA Power Networks said it had to disconnect about one third of those customers due to fire risks with all but 1400 reconnected to the grid by Saturday morning.
In Victoria, there were 809 requests for assistance to the State Emergency Service in the 24 hours to 11am, most for trees down and building damage after the state experienced wild winds on Friday afternoon.
A cool change has brought relief to the southern states with a maximum of 17C forecast in Adelaide and 19C in Melbourne for Saturday.
Warnings are still in place further north, with the NSW Rural Fire Service issuing total fire bans for five state districts including Sydney.
Saturday is expected to be the hottest day of the NSW heatwave which is notable for its spread across the state.
We're expecting hot temperatures, with maximums nearing or reaching 40C for many on Saturday; 40C+ inland. Milder along the coast on Sunday with showers and storms moving through, but remaining hot west of the divide. — Bureau of Meteorology, New South Wales (@BOM_NSW) pic.twitter.com/aDY1s7QGD7December 8, 2023
"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 the coast.
"This is not just the Sydney area," she said.
"All the way up to the Newcastle coast, temperatures are pushing 40 degrees."
Pet owners are being asked to make sure they're kept cool. (Brent Lewin/AAP PHOTOS)
Ms Bradbury said the cool change will continue to push up the coast later in the weekend but inland areas won't get much of a reprieve.
"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.
Severe heat is forecast for Queensland's southern interior and parts of the far north, with temperatures well above average elsewhere.
Southern and western Victoria can expect rain on Saturday but it will remain hot in the state's far north and northeast.
12°C