Tens of thousands of holiday makers fled seaside towns on Australia's east coast on Thursday amid advancing bushfires, as military ships and helicopters began rescuing thousands of others trapped by the blazes.

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.

Authorities urged a mass exodus from several towns on Australia's southeast coast, an area hugely popular in the current summer peak holiday season, warning that extreme heat forecast for the weekend will further stoke the fires.

Eight people have been killed by wildfires in the eastern states of New South Wales (NSW) and Victoria since Monday and 18 are still missing, officials said on Thursday.

Thousands of people had already been evacuated from the greater adjoining region of East Gippsland in Victoria, one of the largest evacuations in the country since the northern city of Darwin evacuated more than 35,000 people in the aftermath of Cyclone Tracy in 1974.