Thousands of tourists have been given less than 48 hours to evacuate fire-ravaged coastal communities as Australia braces for a weekend heatwave expected to fan deadly bushfires.

In this image made from video, officials block the Princes Highway as wildfires approach in South Coast, New South Wales Australia, Tuesday, Dec. 31, 2019. (File photo by AP)

Catastrophic blazes ripped through swathes of the continent's south-east on New Year's Eve, killing at least eight people and stranding holidaymakers as seaside towns were ringed by flames.

The New South Wales (NSW) Rural Fire Service on Thursday morning declared a "tourist leave zone" stretching about 200 kilometres (125 miles) from the popular holiday spot of Bateman's Bay along the usually picturesque south-east coast to neighbouring Victoria state, where people are also being urged to evacuate fire-threatened areas to avoid further loss of life.

The last deaths raise the toll to at least 18 in one of Australia's most devastating bushfire seasons yet.

Visitors are being warned to leave affected areas before Saturday, when another heatwave is expected to sweep across the country to bring gusting winds and temperatures soaring above 40 degrees Celsius (104 degrees Fahrenheit).

That weather will create dangerous fire conditions that officials say will be as bad -- if not worse -- than Tuesday, Australia's deadliest day of fires in a months-long bushfire crisis.

- Queue of cars -

Many tourists and residents spent two nights isolated with no electricity or telecommunications, before authorities on Thursday deemed some roads out of the region temporarily safe to use.

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

NSW Rural Fire Service deputy commissioner Rob Rogers said firefighters were 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 residents 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."

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

- Five feared missing -

There are also mounting fears for five people still missing after the most recent blazes while the number of homes confirmed destroyed has topped 400, with that number expected to rise as firefighters reach devastated communities still isolated by flames.

Navy ship HMAS Choules arrived early Thursday in Mallacoota -- where people huddled on the foreshore for hours as a fire bore down on the remote town -- and would begin evacuating people, a defence force spokeswoman told AFP.

Emergency Management Victoria deputy commissioner Chris Stephenson said some "vulnerable" people had already been rescued and a further 500 people would be extracted in the coming hours.

"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) -- 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.