Temperatures soared past 40C as a heatwave blanketed the region, creating some of the most dangerous bushfire weather since the "Black Summer" blazes of 2019-2020.

Dozens of rural hamlets in the state of Victoria were urged to evacuate while they still could, while three people, including a child, were missing inside one of the state's most dangerous fire grounds.

"If you don't leave now, it could result in your life being lost," Emergency Management Commissioner Tim Wiebusch told reporters.

Powerful wind gusts temporarily grounded firefighting aircraft trying to contain some 30 different blazes dotted across the state.

Already, firefighters fear at least 20 houses have burnt to the ground in the small town of Ruffy, about two hours' drive north of state capital Melbourne.

Country Fire Authority boss Jason Heffernan said the fire danger was "catastrophic" -- the most severe rating possible.

"Victorians should brace themselves for more property loss or worse.

"Today is going to be quite a hectic and volatile day for firefighters, fire authorities and communities."

One of the most destructive bushfires has already razed some 28,000 hectares (70,000 acres) near the town of Longwood, a region cloaked in native forests.

"Some properties have lost everything," said local fire captain George Noye.

"They've lost their livelihoods, they've lost their shearing sheds, livestock, just absolutely devastating," he told national broadcaster ABC.

"But thankfully, at the moment, no lives have been lost."

The worst bushfires have so far been confined to sparsely populated rural areas where towns might number a few hundred people at the most.

- 'Black Summer' -

Photos taken this week showed the night sky glowing orange as the fire near Longwood -- north of state capital Melbourne -- ripped through bushland.

"There were embers falling everywhere. It was terrifying," cattle farmer Scott Purcell told the ABC.

Another bushfire near the small town of Walwa crackled with lightning as it radiated enough heat to form a localised thunderstorm, fire authorities said.

Hundreds of firefighters from across Australia have been called in to help.

"Today represents one of the most dangerous fire days that this state has experienced in years," said state premier Jacinta Allan.

Allan urged people to flee rather than stay put and try to save their homes.

"You will simply not win against the fires of these magnitudes that are created on days like today."

Millions of people in Australia's two most populous states -- Victoria and New South Wales -- are sweltering through the heatwave, including in major cities Sydney and Melbourne.

Power outages left more than 30,000 houses without electricity on one of the hottest days to hit Victoria in years.

Hundreds of baby bats died earlier this week as stifling temperatures settled over the neighbouring state of South Australia, a local wildlife group said.

The "Black Summer" bushfires raged across Australia's eastern seaboard from late 2019 to early 2020, razing millions of hectares, destroying thousands of homes and blanketing cities in noxious smoke.

Australia's climate has warmed by an average of 1.51C since 1910, researchers have found, fuelling increasingly frequent extreme weather patterns over both land and sea.

Australia remains one of the world's largest producers and exporters of gas and coal, two key fossil fuels blamed for global heating.