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Smoke billowing from an out-of-control bushfire has engulfed Hobart, smothering the city in smog.

Authorities have warned that the fire burning in Tasmania's Wilderness World Heritage Area may threaten towns on Friday. A total fire ban is in place in the state.

Plumes of smoke have forced the diversion of flights, sending the airport into chaos. 

The blaze has already torn through 8,800 hectares, double the area the Parks and Wildlife Service had hoped to limit it to.

Smoke billowing from an out-of-control bushfire has engulfed Hobart, smothering the city in smog

Authorities have warned that the fire burning in Tasmania's Wilderness World Heritage Area may threaten towns on Friday

Plumes of smoke have forced the diversion of flights, sending the airport into chaos

A flight due to arrive at Hobart airport was forced to divert to Launceston due to severe weather conditions.

The Tasmanian Fire Service has upgraded the fire's status saying there is a 'very high risk' it will threaten the towns of Maydena, Tyenna, Mount Field and National Park.

Embers, smoke and ash from the fire could threaten homes, and residents have been advised to stay vigilant.

A total fire ban has also been declared for the eastern half of Tasmania on Friday.

About 600 campers had been evacuated from Mount Field National Park, the Mercury reported.

Cafe owner Rachel Power said those on the ground were concerned about the fire spreading.'

'This morning we've been on alert just watching and waiting . . .but within an hour the smoke had really changed colour, the sky had become very red,' Ms Power told the ABC.

'We knew at that point it was time to leave.'

The Parks and Wildlife Service has brought in two air tankers from interstate to dump retardant foam on the fire, state fire manager Paul Black said.

The blaze has already torn through 8,800 hectares, double the area the Parks and Wildlife Service had hoped to limit it to

A flight due to arrive at Hobart airport was forced to divert to Launceston due to severe weather conditions

'There's both the environmental impacts of using materials like that but there's also the cost of it was well - it's quite an expensive exercise,' Mr Black said.

'Overall, when you weigh up the potential impacts of not using those products it far outweighs the minor localised impacts of using them.'

People have been advised to follow their bushfire survival plans and prepare to leave or defend their homes.

John Youd, who owns Tyenna River Cottage, told AAP the sky above Tyenna was 'red and angry' but there was some blue poking through.

'There's a lot of ash, my car is covered in it,' he said.  

Gordon River Road at Maydena is closed to west-bound traffic. 

Popular national park tracks at Mt Field, Freycinet and Maria Island have been closed due to the extreme heat.

Friday marks the sixth anniversary of Tasmania's Dunalley bushfires that destroyed hundreds of homes in the state's southeast.

The temperature at Hobart International Airport peaked at 36 degrees but that dropped markedly in the afternoon thanks to a cool change that will move up the east coast. 

The Tasmanian Fire Service has upgraded the fire's status saying there is a 'very high risk' it will threaten the towns of Maydena, Tyenna, Mount Field and National Park