NSW is bracing for a summer scorcher with temperatures likely to hit the mid-40s and residents urged to keep a check on vulnerable friends and neighbours.

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 also 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 in on the elderly and keep pets cool.

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

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 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 the coast.

“This is not just the Sydney area,” she said.

“All the way up to the Newcastle coast, temperatures are pushing 40 degrees.”

Ms Bradbury said the cool change will continue to push up the coast later in the weekend but inland areas won’t see 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.

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