The Gell River fire, west of Hobart, had on Monday burned through more than 20,000 hectares of bush including a small section of a timber plantation.

"We're hoping to contain the fire in the coming weeks, however it will take some time to completely extinguish this fire," Tasmania Fire Service operation officer Mark Klop told reporters.

The fire poses no risk to communities but an advice warning remains in place for the towns of Maydena and Tyenna, and Mount Field National Park.

A smoke alert is in place for Hobart and surrounding areas.

The fire, sparked by lightning strikes 10 days ago, is putting pencil pines, king billy pines, peat soils and cushion plants with world heritage value at risk

Mr Klop said mild weather over the coming week would help crews get a handle on the fire but rain was needed to put it out.

Specialist firefighters have been flown by helicopter to fight the blaze which can't be reached by road.

"It's a very challenging fire due to its location. We don't have the luxury of driving to the fire. We are heavily reliable on aircraft," Mr Klop said, adding waterbombing efforts were ongoing.

"Some nice, heavy rain would be really, really good."

A small part of a state-government-owned Sustainable Timber Tasmania plantation was burned, with the damage to be known in coming days.

A sprinkler line deployed at Lake Rhona has thus far protected sensitive vegetation.

Australian Associated Press