New South Wales Premier Gladys Berejiklian said her state is facing "another terrible day" and called on people in areas threatened by the fires to leave while they can.
"I'm pleased to say that we've never been as prepared as we are today for the onslaught we're likely to face," Berejiklian told reporters at a news conference Saturday morning. "All of the major road networks are still open but we can't guarantee that beyond the next few hours. So there are still windows for people to get out."
Two people were confirmed dead in a blaze on Kangaroo Island off the coast of South Australia, bringing the overall nationwide death toll this summer to at least 21.
The fire broke containment lines Friday and was described as "virtually unstoppable" as it destroyed buildings and burned through more than 14,000 hectares (35,000 acres) of Flinders Chase National Park. While the warning level for the fire was reduced Saturday, the Country Fire Service said it was still a risk to lives and property.
New South Wales Rural Fire Service Deputy Commissioner Rob Rogers warned the fires could move "frighteningly quick." Embers carried by the wind had the potential to spark new fires or enlarge existing blazes.
"We are unfortunately very likely to lose homes but we will be very happy and call it a success if no lives are lost," he said.
Rural Fire Service Commissioner Shane Fizsimmons said the 264,000-hectare (652,000-acre) Green Wattle Creek fire in a national park west of Sydney had the potential to spread into Sydney's western suburbs.
"There is potential for the fire to break out, cross the (Warragamba) dam and move into the western suburbs of Sydney," he said. "That fire is burning. It has the potential to come out into more populated areas this afternoon."
He said crews have been doing "extraordinary work" by setting controlled fires and using aircraft and machinery to try to keep the flames away.
Fitzsimmons called on residents and tourists in the path of the fires to evacuate as soon as possible.
"Our message has been to make sure you leave yesterday," he said. "Leaving it until today is cutting it fine. The sooner you make that decision the better and I would say do it now. Don't leave it any longer because the window will shrink and will shrink very quickly."
More than 130 fires are burning in New South Wales and at least half of those are out of control. Temperatures in parts of the state are expected to soar in the mid-40s Celsius (about 113 Fahrenheit) amid strong winds and low humidity.
A total of 48 fires were burning across almost 320,000 hectares (791,000 acres) in Victoria state and conditions are expected to worsen with a southerly wind change.
"We still have those dynamic and dangerous conditions, the low humidity, the strong winds and, what underpins that, the state is tinder dry," Victoria Emergency Services Commissioner Andrew Crisp said.
Thousands have already fled fire-threatened areas in Victoria and Crisp urged more people to leave.
"If you might be thinking about I can get out on a particular road close to you, well there's every chance that a fire could hit that particular road and you can't get out," he said.
Victoria police reported heavy traffic flows on major roads and praised motorists for their patient and orderly behavior.
The early and devastating start to Australia's summer wildfires has already burned about 5 million hectares (12.35 million acres) of land and destroyed more than 1,400 homes. That's more acres burned in Australia than any one year in the U.S. since Harry Truman was president.