More than 30 homes have been razed in the worst fire conditions Australia has seen since a 2009 inferno killed 173 people, with flames threatening the country's second-largest city, officials said Monday.

Hot, dry winds and soaring temperatures fanned scores of major blazes across the southeast on Sunday, with Victoria state sweltering through its worst fire risk weather in five years.

"They were ferocious fires, they ran hard, they hit homes," said fire commissioner Craig Lapsley.

The emergency came exactly five years after the so-called Black Saturday firestorm devastated the state, flattening whole towns in what was Australia's deadliest natural disaster of the modern era.

Victoria state premier Dennis Napthine said it had been the worst fire danger day since Black Saturday, with more than 30 homes confirmed lost so far across the state.

"At this stage we have no evidence of loss of life, which is a great effort by the firefighters and all emergency services, and at this stage we have no evidence of serious injury," Napthine said.

Prime Minister Tony Abbott said the government stood ready "to do all we reasonably can to ensure that people get the help they need in these difficult, difficult hours and days".

Hundreds of people spent the night in evacuation shelters after being forced to flee their homes.

A major open-cut coal mine was ablaze, with a nearby power station in the path of one fire. Napthine said emergency crews were working "very, very hard in that area to protect those strategic assets".

Six blazes remained at emergency level Monday including a 40-kilometre (25-mile) front on the outskirts of Melbourne, Australia's second-biggest city, with tens of thousands of hectares scorched.

At least 12 of Sunday's fires were thought by police to have been deliberately lit.

Residents described houses "exploding" into flames, with one woman saying it was like "the whole world was alight".

Fire crews said they saved about 550 properties from an intense blaze at suburban Keilor near Melbourne Airport.

"It was very hard for us to initially get that fire to stop due to the fact that the winds were blowing up towards the airport," said Rob Purcell from the Metropolitan Fire Brigade.

"But we had great firefighters who did a brilliant job in saving a huge amount of assets."

'Terrifying' experience

At Warrandyte, also on the outskirts of Melbourne, resident Brenda Ireland described a "huge ball of flames just rushing across our backyard".

Neighbour Jamie Conlon said he "just sprinted" shirtless and without shoes down his driveway, which was flanked by flames.

"I didn't stop to look... I thought I was dead," he said. "I was just screaming. I was terrified."

One man told the Australian Broadcasting Corporation that fast-moving flames had cut off his evacuation route, forcing him to stay and defend his property near Forbes north of Melbourne.

"It was a massive wall of fire," said the man, who identified himself only as Peter.

"I really didn't think I was going to make it but luckily in our case the fire spread around our house and our shed, went either side of us. And then for a good five, 10 minutes I thought 'This is it'."

Chintin resident Helen Goudy described abandoning her house as flames licked the walls and running 20 metres to a neighbour's dam with her children aged 2, 4 and 7. They took refuge under a blanket for an hour while the flames passed.

The fire threat eased in many areas Monday as cooler temperatures arrived from the south.

Vast wildfires are common in Australia's December-February summer months. Some 25 homes were destroyed in the southeast last month following a week-long heatwave that also saw 56 homes razed in separate blazes on the west coast.

Experts say heatwaves are becoming longer and more frequent in Australia due to climate change.