The worst blazes continue to burn in Victoria's east and on the NSW south coast, where eight people have died, others are missing and hundreds of homes have been destroyed.

More than 150 fires continue to burn across the two states as the predicted catastrophic fire conditions force thousands to evacuate "tourist leave zones" by road, air and sea.

"It is important, as we work through those evacuations, that people continue to remain patient, remain calm," Prime Minister Scott Morrison told reporters on Thursday.

"I understand the anxiety and I understand the fear that is there for many and I understand the frustration, but this is a natural disaster."

The NSW government has declared the third state of emergency of the bushfire season taking effect from Friday with temperatures in excess of 40C and strong winds forecast for Saturday.

"We're going to have a very long, difficult day of hot, dry winds, dominating out of the north, northwest before a southerly change," NSW Rural Fire Service Commissioner Shane Fitzsimmons warned.

"It is going to be a very dangerous day. It's going to be a very difficult day."

Power cuts, patchy telecommunications, food queues and water shortages continue to frustrate residents and holidaymakers in isolated communities up and down the coast.

Roads remain blocked as police help convoys of cars thread their way through bushfires to escape and the Defence Force begins relief operations in Victoria's fire-savaged East Gippsland using Black Hawk helicopters and naval vessels.

Premier Daniel Andrews confirmed that 17 people are missing in the region with grave fears for their safety as authorities urged people to leave before the weather worsens.

"I can't stress enough, these types of conditions that we are facing again in our state," Victorian Assistant Emergency Services Commissioner Deb Abbott said.

"These fires have already caused significant damage. They have caused significant loss and our community needs to act now."

The warning came as the CFA declared a total fire ban across much of Victoria on Friday and the navy prepares to evacuate 800 people trapped in Mallacoota.

The town was hit by a massive blaze on Tuesday as 4000 people sheltered on a beach amid apocalyptic scenes that were broadcast across the globe.

NSW had not yet requested ADF support for evacuations but Mr Morrison said naval assets were ready to provide support

"Our task has been to fully support and provide whatever assistance is necessary through all the various agencies of the Commonwealth," he said.

More $21 million in commonwealth disaster payments has been paid to fire victims in NSW.

"We expect more of that to continue in Victoria as the full devastation of the fires there becomes more evident," Mr Morrison said.

Treasurer Josh Frydenberg said it's too early to say what the full financial cost of the fires will be to taxpayers but stressed a dollar figure is not the government's focus.

"Our primary concern is not about the financial cost. It's about the human cost of these tragic fires," he said.

In South Australia, where 88 homes have been lost this summer, authorities fear fires could flare with temperatures in parts of the state forecast to nudge 50C.

More than 30 fires are burning in Western Australia where 150 holidaymakers and truckies remain stranded at a remote roadhouse on the Nullarbor with the Eyre Highway still cut.

One home has been destroyed in Tasmania with fire burning close to others in Hobart's northwest late on Thursday afternoon.

The ACT has declared a state of alert as toxic smoke blankets the capital.

Across Australia, 17 people have died and more than 1400 homes have been destroyed in bushfires this fire season.

Australian Associated Press