Arsonists were also relighting fires that had been brought under control, a fire authority official in Victoria said.
Steve Warrington, a deputy chief of firefighting operations, told local radio: "We know we do have someone who is lighting fires in this community.
"While we often think it is spotting [embers spreading flames], we also know that there are people lighting fires deliberately."
"Some of these fires have started in localities that could only be by hand, it could not be natural causes," Kieran Walshe, the Victoria state police deputy commissioner, said on Sunday.
Firefighters and army units are struggling to get control over the blazes across the states of Victoria, South Australia and New South Wales.
Caroline Douglass, a spokesperson for the country fire authority, told Al Jazeera: "We still have roughly 25 [fires] going. We currently have over 4,000 firefighters in the state who are working away.
"The army will assist us with that and we are asking for assistance from other states in Australia and they are also helping us with that too."
Witnesses described seeing trees exploding and ash raining from the skies.
Lance Sheppard, a resident of Healesville in Victoria, told Al Jazeera: "One minute we were looking at a whole range of smoke coming out of the northwest. And the next ... the weather turned and within minutes it was on us.
"It was like a roaring jet coming at us ... There was no way we could get away, but we had a fire plan put in place, we had hoses everywhere and a fire pump and that's what saved us."
Kevin Rudd, Australia's prime minister, toured the burned-out region on Sunday, and said: "Hell in all its fury has visited the good people of Victoria in the last 24 hours."
The southeastern Australian states have been gripped in a heatwave for the past two weeks.
Fierce winds were fanning the fires and pushing them in unpredictable directions in Victoria on Sunday, after temperatures reached a state record of 47 degrees Celsius.
Forecasters said hot and uncertain weather conditions would continue through the day on Sunday.
Blair Trewin, a climatologist with the National Climate Centre in Melbourne, told Al Jazeera: "They are the most extreme conditions that we have ever seen in historic record in parts of southeastern Australia.
"We are seeing an upward trend in temperatures in Australia, as elsewhere in the world."
The worst wildfires in recent memory killed 75 people and razed 2,500 homes in Victoria in 1983.