Winds, storms and heatwaves goad 'erratic' fire threat

Australians are on high alert as a widespread heatwave, powerful winds and lightning create the perfect storm for dangerous fires.

Temperatures topped 40C in parts of Sydney and elsewhere in NSW on Saturday, with heatwave warnings issued for multiple heavily populated areas.

Many residents headed to beaches and waterways to cool off amid the sweltering conditions.

Central and western parts of Queensland and areas in Western Australia's north also sweated through balmy conditions.

But the areas of greatest concern were in the NSW Hunter and mid-north coast regions, where windy conditions were expected to fan existing fires and any potential flames.

"We're on the boundary between sea breezes ... and those hot, west to northwesterly winds that can cause erratic and variable fire danger," Bureau of Meteorology senior meteorologist Dean Narramore told ABC News.

A total fire ban is 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.

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, 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 will temper the island state on Saturday, bringing relief and even snow to some parts of Tasmania.

"We've seen more friendly conditions for crew to get in and start working on containing the fire edge," Tasmania Fire Service's Simon Pilkington told reporters.

"It's a sensitive ecosystem, so there's natural and cultural values we're trying to protect."

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 next week, according to the weather bureau.

"We're in that cycle now with heat just continuing to move across the country," Mr Narramore said.

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.