Smoke billows during bushfires in Buchan, Victoria, Australia. Glen Morey/Reuters

Two people died on Saturday as Australia braced for one of the worst days in its wildfire crisis, with strong winds and high temperatures forecast to bring flames to more populated areas, including the suburbs of Sydney.

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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."

A burnt out car destroyed by bushfires is seen just outside Batemans Bay in New South Wales on Thursday. AFP

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.

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"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.”

Queue of cars

Many tourists and residents spent two nights isolated with no electricity or telecommunications, before authorities on Thursday declared some roads safe to use.

NSW Transport Minister Andrew Constance called it the "largest evacuation of people out of the region ever", with a queue of cars stretching along the highway toward Sydney as thousands flee the area.

Pedestrians wear masks as smoke shrouds the Australian capital of Canberra, Australia, on Thursday. Mark Baker/AP

One driver told the media it had taken her three hours to travel just 50 kilometres (30 miles).

NSW Rural Fire Service deputy commissioner Rob Rogers said firefighters would be unable to extinguish or even control the raging blazes.

"The message is we've got so much fire in that area, we have no capacity to contain these fires," he told ABC.

"We just need to make sure that people are not in front of them."

John Steele, 73, who lives outside the south coast town of Merimbula, told AFP some people were "panicking" amid the warnings to evacuate.

"There's so much misinformation on Facebook and on the web," he said.

Steele said the region had been "chaotic" in recent days as fresh produce and fuel supplies ran low, but he and his wife were staying put for now.

"We're happy to see every man and his dog leave town," he said. "We are cautious, we have our bags packed."

Cars line up to leave the town of Batemans Bay in New South Wales to head north on Thursday. Peter Parks/AFP

Authorities still have not been able to reach some rural communities, such as the town of Genoa in Victoria.

'Weeks' to evacuate

The number of homes confirmed destroyed in recent days has topped 400, with that figure expected to rise as firefighters reach communities still isolated by flames.

Navy ship HMAS Choules arrived early on Thursday in Mallacoota — where people huddled on the foreshore for hours on New Year's Eve as a fire bore down on the remote town — to begin evacuating people, a defence force spokeswoman said.

Emergency Management Victoria deputy commissioner Chris Stephenson said some vulnerable people had already been rescued and a further 500 people would be extracted at the beginning of a lengthy operation.

"Today it's starting to move potentially the tourists in Mallacoota, and residents who may not want to be there," he said.

"With the number of people in Mallacoota, this is going to take days, if not weeks, to be able to achieve."

Military aircraft have also been working with emergency crews to drop relief supplies into isolated areas and assess the fire damage.

This season's blazes have destroyed more than 1,300 homes and scorched over 5.5 million hectares (13.5 million acres) across the country -- an area far greater than Denmark or the Netherlands.

The unprecedented crisis has sparked street protests calling on the government to immediately act on climate change, which scientists say is creating a longer and more intense bushfire season.

Conservative Prime Minister Scott Morrison has come under increasing pressure for his actions, which included holidaying in Hawaii as the disaster unfolded and reiterating his support for Australia's lucrative -- but heavily polluting -- coal mining industry.

In his first official press conference since the latest blazes flared, Morrison said Thursday said "every absolute effort" was being made to assist affected communities.

"The best way to respond is the way that Australians have always responded to these events and that is to put our confidence in those who are fighting these fires," he said, while defending Australia's climate change policies as "sensible".

Agencies