Large parts of Australia are experiencing extreme weather with people urged to stay indoors during the hottest part of the day before the cool change sets in.

NSW is enduring a summer scorcher with temperatures in the mid-40s and residents urged to keep a check on vulnerable friends and neighbours.’

Penrith and Richmond in Sydney’s outer west were sweltering in 40C degree heat at midday on Saturday with a forecast top of 44C on the way and 40C for the city.

Inland will also be hot and windy, with the ACT headed for a maximum of 37C.

Heatwave warnings are current 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.

Hot conditions are set to continue across NSW today, driven by warm north-westerly winds.

A Severe Heatwave Warning is current for much of the state, with many locations forecast to reach 8-10+ degrees above average.

That prompted a warning to 100,000 music fans heading to Sydney’s Olympic Park precinct to watch the Foo Fighters, 50 Cent and the EPIK indoor music festival.

The area is tipped to reach 38C, with authorities urging concertgoers to allow extra time to travel in the heat and seek out shade while waiting for transport.

Saturday is expected to be the hottest day of the NSW heatwave which is notable for its spread across the state.

The Bureau of Meteorology’s Mariam Bradbury said the intense conditions were coming across the Dividing Ranges and even a sea breeze would not spare coastal areas.

”This is not just the Sydney area,” she told AAP.

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

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.