Fuelled by searing temperatures and high winds, more than 200 fires are burning across the southeastern states of New South Wales (NSW) and Victoria, threatening several towns.

The NSW state government declared a state of emergency, beginning on Friday, giving authorities the power to forcibly evacuate people and take control of services.

"It is hell on earth. It is the worst anybody's ever seen," Michelle Roberts said by telephone from the Croajingolong Cafe she owns in Mallacoota, a southeastern coastal town where 4,000 residents and visitors have been stranded on the beach since Monday night.

Roberts hoped to get her 18-year-old daughter onto a naval ship, which arrived off the town on Thursday, in order to escape the fires and thick smoke engulfing the town.

The HMAS Choules is expected to make two or three voyages over the coming days, state authorities said.

Elsewhere, long queues formed outside supermarkets and petrol stations as residents and tourists sought supplies to either bunker down or escape the fires, emptying shelves of staples like bread and milk.

More than 50,000 people were without power and some towns had no access to drinking water.

"Everyone's just on edge," said Shane Flanagan, a resident of Batemans Bay on the NSW coast.

Authorities urged a mass exodus from several towns on the southeast coast, an area popular with tourists during the summer holiday season, warning that extreme heat forecast for the weekend will further stoke the fires.

"The priority today is fighting fires and evacuating, getting people to safety," Prime Minister Scott Morrison told reporters in Sydney. "There are parts of both Victoria and New South Wales which have been completely devastated, with a loss of power and communications."

Eight people have been killed by wildfires in NSW and Victoria since Monday and 18 are missing, officials said on Thursday.

Temperatures are forecast to soar above 40 degrees Celsius (104 degrees Fahrenheit) along the south coast on Saturday, bringing the prospect of renewed firefronts to add to the around 200 current blazes.

"It is going to be a very dangerous day. It's going to be a very difficult day," NSW Rural Fire Service (RFS) Commissioner Shane Fitzsimmons said.

REUTERS