Fire crews working on a large blaze at Gell River in Tasmania's south-west wilderness area are hoping to bolster containment lines before predicted hot weather on the weekend.

Key points: Smoke from the fire is expected to affect Hobart and surrounds in the coming days

Smoke from the fire is expected to affect Hobart and surrounds in the coming days The blaze is being fought by crews on the ground and in the air

The blaze is being fought by crews on the ground and in the air Authorities say a big rain event is what is needed to extinguish the fire conclusively

The fire came dangerously close to threatening ancient vegetation at Lake Rhona but authorities have credited a new sprinkler system with protecting the area.

Remote area firefighters and aircraft are now focusing on containing the south-western edge of the blaze, which has now burnt in excess of 20,000 hectares.

The fire also caught the south-east corner of forestry plantations worth hundreds of millions of dollars, but Sustainable Timbers Tasmania said it cannot assess the damage until the blaze is under control.

The Gell River fire began as two separate lightning strikes 10 days ago.

It now has a 90-kilometre perimeter, with infrared imagery taken by helicopters showing spot fires and flames burning up mountainsides.

Terrain at Lake Rhona has been burnt but most of the sensitive vegetation has been saved so far. ( ABC News )

Parks and Wildlife Service (PWS) deputy incident controller Nic Deka said sprinklers at Lake Rhona halted the threat at Lake Rhona for now.

"It was a good sight because the protection works that we've done there with the sprinkler systems have been very effective at halting the fire and protecting the higher value vegetation in that particular site," he said.

"There's pencil pines, there's cushion plants, there's species that are typical of alpine vegetation types."

About 60 crews are fighting the blaze, with 40 personnel stationed at nearby Strathgordon.

"Getting crews onto the fire ground as early in the day is critical because that's when you get the best conditions for containing and supressing fire," he said.

"Because of the relatively benign weather over the next two to three days, the work we do will be critical as we work into next weekend when the weather is expected to warm up a bit."

Sprinklers are being trialled as part of efforts to protect environmentally sensitive areas in Tasmania's south-west. ( Supplied: Parks and Wildlife )

Wilderness conservationist Geoff Law has concerns about what is at stake if the weather changes.

"East of the fire are some majestic tall eucalypt trees, eucalyptus regnans are the tallest flowering plant on earth, and that particular area has one of Australia's largest stands of unlogged, untouched old growth eucalypt regnans," he said.

"And this fire is a dire threat to that stand of ancient majestic trees."

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He believed lessons have been learnt from a world heritage area blaze in 2016 when large tracts of high-value wilderness was destroyed.

"This fire is being taken very very seriously, clearly there have been a lot of crews, there a lot of aerial crews have been deployed," he said.

"It does seem some lessons have been learned but we won't know all the answers until the fire has been extinguished."

He is relieved Lake Rhona has so far been spared.

"These glacial lakes are extraordinarily beautiful, they are occur next to jagged mountains, there's beach there and ancient eucalypt trees. For that area to be destroyed would be tragic," he said.

About 60 crews are now tackling the Gell River blaze. ( ABC News )

The Tasmania Fire Service (TFS) said there was no immediate threat to the small communities in the upper Derwent Valley, but that the greater Hobart area was likely to be affected by smoke in the coming days.

Despite favourable weather conditions over the next few days, TFS incident controller Rod Sherrin said crews were preparing for a drawn-out battle in difficult terrain.

"It's a large fire on the landscape and history will tell us it will take some considerable time to bring it to a conclusion," he said.

A temporary bridge has been placed across the Florentine River to access the plantation so large earthmoving machinery can move into the area and bolster containment lines.

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Rain needed to reduce risk

Fire ecologist Grant Williamson said smoke would continue to be an issue.

"The impact of smoke at the moment is going to be the major worry," he said.

"There's no severe fire weather forecast over the coming period. Certainly if the fire continues to burn over the coming weeks that could happen again."

He said lower wind speeds and temperatures over the coming days would help firefighting efforts but a big rain event was needed.

Sorry, this video has expired Aerial vision and infrared imagery shows the fire burning in Tasmania's south west (NSW Rural Fire Service)

"[The fire] is going to have difficultly getting into those wet forests, they don't usually burn under these sort of mild weather conditions so hopefully that gives the opportunity for TFS and Parks to get it under control," he said.

The speed of the response has been the subject of criticism from environmentalists and a union representing PWS workers — with Emergency Services Minister Michael Ferguson hitting back, describing the comments as "pathetic".

But Mr Williamson said the response was about managing priorities, with authorities deeming it important to have crews in reserve.

"There was a severe fire weather forecast and they have to balance fighting wilderness fires with having resources available around populated areas around Hobart," he said.

"On Friday we saw a lot of small fires cropping up around Hobart and the surrounds, so it's important to have crews available for those as well."

Mr Sherrin said the speed of the response was appropriate, describing the activities of PWS as "admirable".

"It was in a difficult spot to get to, it was a lightning strike, they had waterbomber aircraft as soon as practicable and as soon as they could they inserted their remote area teams to do some active firefighting," he said.

"They were actively fighting on that fire prior to the total fire ban day," Mr Sherrin said.

As the fire is affecting a national park, the State Government can make a claim for the expenses associated with fighting the blaze to the Federal Government — but it would need to demonstrate that the costs incurred for the disaster were extraordinary.