Emergency warnings were issued for residents near fires on the NSW Central Coast and in the Hunter region on Saturday as dozens of blazes burned across the state.
At least 12 homes were believed destroyed by a bushfire at Koolewong, not far from built-up areas on the Central Coast.
Several more properties were thought to be affected at Buladelah, north of Newcastle, although fire assessors were yet to survey the full extent of the damage.
Firefighters were quick to respond to the Koolewong blaze, but by the time trucks arrived several homes were already in flames, NSW Rural Fire Service assistant commissioner Leigh Pilkington told ABC TV.
After preventing disaster for dozens more nearby homeowners, emergency services were working to ensure the blaze didn't reach the larger hub of Woy Woy.
The federal MP for the region, Gordon Reid, said it was devastating that people had lost their homes in the disaster.
"That's people's lives, that's people's livelihoods," he said.
Temperatures topped 40C in parts of Sydney and elsewhere in NSW on Saturday, with heatwave warnings issued for multiple heavily populated areas.
The mercury in some inland areas neared 44C.
The Hunter and mid-north coast regions were of particular concern for firefighters, with windy conditions expected to fan existing fires and any potential flames.
The Milsons Gully fire in the upper Hunter also reached emergency level on Saturday.
"There are homes within the area," RFS spokesman Greg Allan told AAP, but the service was not able to provide more details as conditions were fickle.
A total fire ban was in place across Sydney and many parts of central NSW as forecast thunderstorms risk sparking more fires, with winds of up to 90km/h expected in some areas.
Hot, west-to-northwesterly winds could cause erratic and variable fire danger, Bureau of Meteorology senior meteorologist Dean Narramore said.
"Dry lightning is also a risk with some of our storms through NSW and that could ignite more fires across the state," he said.
The state's inner north between Cobar and Tamworth had the highest thunderstorm risk, including damaging winds and hail.
A cool change moving through Victoria and southern NSW is not expected to bring relief to the Central Coast and northern NSW until late Sunday morning and into the afternoon.
Central and western parts of Queensland and areas in Western Australia's north also sweated through balmy conditions.
Elsewhere, more than a dozen homes and shacks, outbuildings and cars were damaged and two firefighters were injured fighting an out-of-control fire at Dolphin Sands on Tasmania's east coast on Friday.
Crews in Tasmania are continuing to investigate its cause as they deploy air and ground tankers to contain the fire, which has burnt more than 700 hectares.
While alerts for the blaze have been downgraded, the Tasmania Fire Service has urged people not to return.
Some residents would be without power for a week after TasNetworks confirmed severe fire damage to dozens of power poles in the Dolphin Sands area.
But cooler temperatures and showers tempered the island state on Saturday, bringing relief and even snow to some parts of Tasmania.
While temperatures were expected to ease from Sunday for most of NSW and southern Australia, the heat is expected to build across northern Australia and WA, returning to much of the country early in the coming week.
Victorians faced milder temperatures after a cold front and thunderstorms followed hot, dry, windy weather on Friday.
Firefighters were working to contain a bushfire in Markwood, 280km northeast of Melbourne, which has affected at least three properties.
West Australians in parts of the state's southern central areas have been urged to monitor conditions after bushfires were reported at Wallaroo, Barragup and Nambeeluop.
18°C