Bushfires engulfing Australian Tasmania could have detrimental long-term effects on the state's unique world heritage forests, according to conservationists.

Fires continued to burn out of control across Australia's southernmost state on Thursday, with the north-west of the island state under greatest threat.

Conservationists have expressed fears that fires in the world heritage-listed central plateau could wipe out 1,000 year-old trees.

"In my opinion - and I've been working on these issues since 1981 - it's the gravest crisis the world heritage area has faced in this time," former Wilderness Society conservationist Geoff Law told Guardian Australia.

Law said the rarity of bushfires in the central plateau region - perhaps once a millennium - meant local fauna had not evolved sufficiently to live in a cycle of renewal, and that fires had catastrophic consequences for some species.

"It's killing trees that are over 1,000 years old; it's burning up soil that takes over 1,000 years to accumulate," said David Bowman, a professor of environmental change at the University of Tasmania, to Guardian Australia on Wednesday.

"We are in a new place... We just have to accept that we've crossed a threshold, I suspect. This is what climate change looks like."

An estimated 150 residents from communities in the northwest of Tasmania were relocated to safety by boat late on Wednesday evening after seeking shelter on a beach.

Other Tasmanian icons have been hit by bushfires too, with access to Cradle Mountain in the state's central north and popular Overland Track from which hikers were being air lifted to safety closing on Wednesday.

Meanwhile, the small community of Temma remained on high alert on Thursday morning, with the Tasmania Fire Service warning of an "uncontrollable, unpredictable and fast-moving fire" which has already burned approximately 13,000 hectares.

This Australian bushfire season has already claimed six lives and hundreds of homes in Western Australia, South Australia and Victoria.