About 10 per of the Barrington Tops National Park world heritage area was damaged in the recent bushfires, the NSW government confirmed.
The area of affected rainforest - which is part of the ancient Gondwana system - covers 4011 hectares, a government spokesperson said.
In 2010, the National Parks and Wildlife Service predicted in its Barrington Tops plan of management that climate change was "one of the greatest potential threats to the values" of the area.
Nature Conservation Council ecologist Mark Graham said "this might end up being the start of a process of the area drying out".
"This seems to be one of the first phases of these changes of heating and drying. Given these forests have been there for tens of millions of years since the times of Gondwana, that's a frightening prospect," Mr Graham said.
"These forests have continued largely unchanged from when Australia split from Antarctica and South America and an asteroid hit the Yucatan Peninsula in Mexico and made the dinosaurs extinct."
Mr Graham said the Barrington wilderness was "among the most special" forests in the world.
"It hasn't been impacted much by modern humanity. In places, it's pretty much as pristine as you can get."
He said the Barrington was "one of the bigger reserves" of the Gondwana system.
Ancient world-heritage areas stretch in pockets from the Barrington Tops to the Gold Coast. They are the last remnants of the supercontinent Gondwana, which broke up about 180 million years ago. Usually these forests are too wet to burn.
The government spokeswoman said 54 per cent of the NSW parts of the Gondwana world heritage areas had been affected by fire.
"The most affected ecosystems are rainforests," the spokeswoman said.
As well as the Barrington and northern NSW, wet forests also burnt in southern Queensland, highlighting the widespread climate change threat.
Mr Graham said there had been indications that "we would face these types of processes under global heating".
He said there was a great fear that we might be facing a period in which the Gondwana rainforests "may not be able to survive".
"I really hope I'm wrong."
Asked if there were plans to protect the Barrington world heritage areas from fire in future, the government spokeswoman said: "We are planning for the longer-term restoration and recovery of our native animals, plants and landscapes across NSW".
"This includes protecting the remaining areas of unburnt habitat. The NSW government will continue to update our response as we improve our understanding of the impacts of these unprecedented fires."
Mr Graham said the time for action was now. It was essential to "protect unburnt areas at all costs".
"In the coming weeks, months and seasons, we need to get as many people into the forest as we can to take note of where nature is looking after itself, where there aren't weeds and where there are weeds," he said.
He said weeds were now "one of the greatest risks".
Weeds like Scotch broom, lantana and blackberry should not be allowed to advance in the Barrington.
"A very nutritious ash bed supercharges weeds."
Mr Graham said the Barrington rainforest areas had "an amazing array of ancient species".
While some of the trees, shrubs and vines may tolerate a level of fire, some can't because there are "delicate things like ferns and orchids".
"And by ferns I mean the stuff that's on dripping rock faces and the classic rainforest stuff," he said.
"The biggest problem is the impacts upon fauna. A lot of the wildlife that lives within these areas require permanent wet leaf litter and humid areas."
The fires would have reduced the range and numbers of some species.
"It might mean initially there's direct mortality to things like lizards, frogs, invertebrates and snakes."
While the Barrington Tops did receive heavy rain in the past fortnight, the fires may have a lasting effect given the unprecedented nature of the drought.
"It's been so dry that even areas you wouldn't think would burn in your wildest nightmares have been burning," he said.
The drought led to the Barrington River running dry for "the first time in European recorded history". "The areas that haven't burnt are now a lot more exposed to future drying out and fire because that body and mass of wetness that should be there has been reduced," he said.