"It's just going to be, probably by a long way, the worst day ever in the history of the state in terms of temperatures and winds," John Brumby, premier of the state Victoria, said of Saturday's conditions.
"If you don't need to go out, don't go out. It's a seriously bad day."
Weather forecasts say hot, dry winds will blow across the region over the weekend, creating conditions worse than those when a wildfire killed 75 people razed 2,500 homes in 1983.
Blair Trewin, a climatologist with the National Climate Centre in Melbourne, told Al Jazeera that conditions on the ground exceed previous records.
"They are the most extreme conditions that we have ever seen in historic record in parts of southeastern Australia," he said.
"We are seeing an upward trend in temperatures in Australia as elsewhere in the world."
Jeremy McAnulty, a New South Wales health department spokesman, said officials were preparing for heat-related emergencies.
"More people get seriously ill and death rates can increase, so it's really important that people remember ... keep cool, keep well hydrated, look after your neighbours, and rest up,'' McAnulty said.
Most of the fires burning were contained within firebreaks by thousands of volunteers manning hoses and building firebreaks.
On the eastern outskirts of Melbourne, Australia's second largest city, 150 hectares were on fire with flames as high as 10 metres during. Around 140 firefighters were deployed to the area during the night.
Power lines supplying electricity to Melbourne were under threat and people in nearby townships were being urged to either evacuate or prepare to join firefighting efforts.
More than 100,000 firefighters remained on standby in the fire-affected states. Smoke hung over parts the capital Sydney from fires burning in and near the city.
Meanwhile, in sharp contrast to the fires in the country's southeast, much of the northern state of Queensland was flooded and has been declared a disaster zone.
At least 3,000 homes have been affected by the flood.
The sugarcane and cattle industries have been devastated by the floods, with thousands of stranded cows starving to death.