Australia's bushfires have been so devastating, the country's forests may not be able to reabsorb the toxic carbon dioxide produced by the blazes, climate scientists say.

Key points: At least 2.7 million hectares of NSW land, including "exceptionally carbon-dense" forests, have been burnt this bushfire season

At least 2.7 million hectares of NSW land, including "exceptionally carbon-dense" forests, have been burnt this bushfire season Drought and intense blazes have disrupted vegetation's bushfire recovery process

Drought and intense blazes have disrupted vegetation's bushfire recovery process One academic has compared the crisis to the Amazon fires

Bushfires are normally considered to be "carbon neutral" because, unlike fossil fuels, their emissions output is reabsorbed when the vegetation in fire-affected areas regrows.

However, experts fear the sheer scale and intensity of this year's unprecedented fires, coupled with worsening drought conditions, has disrupted this recovery process.

The NSW Department of Planning, Industry and Environment said it was yet to determine size of the fires' carbon footprint.

According to the Global Fire Emissions Database, the fires in the Amazon Rainforest in Brazil — which were burning at a rate not seen in almost a decade — added 14 million tonnes of carbon to the atmosphere this year.

Scientists fear climate change has disrupted the way forests recover after a fire. ( ABC News: Kathleen Calderwood )

Pep Canadell, a senior research scientist at the CSIRO Climate Science Centre, said Australia's forests had more in common with the Amazon than people would think.

"We're talking about very different sizes, of course. [There] are more carbon-dense forests in Amazonia than in Australia," Dr Canadell said.

"Having said that, a lot of [bush areas] burning now in NSW are actually exceptionally carbon-dense — they're very tall and quite dense forests.

"So from a per square metre or hectare level, we're certainly not shy away from what is happening in the Amazon."

Around 14 million tonnes of CO2 have been released by the Amazon rainforest fires this year. ( Reuters: Bruno Kelly )

David Bowman, a professor of environmental change biology at the University of Tasmania, has been worried about this "nightmare scenario" for a decade.

"In a stable climate it's like a bank account, where a fire comes along and burns some forest and releases carbon," he said.

"When the forestry regrows it's like putting money back into your account. Over the years, your bank balance account is about the same."

He said intense fires were like "huge transactions", but the "high mortality rate" of NSW and Queensland forests meant they were not taking back the carbon being withdrawn.

The blazes, he said, had been so savage that even the famously resilient dry sclerophyll and eucalyptus forests were not likely to regenerate effectively.

The drought had already stressed the hardy trees through depriving them of nutrients, leaving them likely to regrow slower and smaller, if at all.

The NSW Rural Fire Service (RFS) said at least 2.7 million hectares, with a 19,235-kilometre perimeter, had been burnt since the beginning of this year's bushfire season.

This map shows what 2.7 million hectares of burnt land looks like in comparison to the rest of the state.

If all this season's bushfires were in one place, this is how much land they would have burnt. ( ABC News )

Dr Canadell said it was difficult to determine if specific fires would be carbon neutral because the regrowth process could take a long time.

But he said decreased rainfall and a lack of remediation of the land degraded by flames and agriculture meant some of the millions of tonnes of carbons from this year's bushfires would remain in the atmosphere.

"If there were no changes in fire frequency over time, it would be carbon neutral," he said.

"[But] carbon neutrality breaks at the point where you start burning more than regrowth can catch up with."

Professor Bowman said we may already have entered a "slippery slope of negative feedback" where forests become sources of carbon instead of carbon sinks.

"The nightmare scenario is that because of climate change, the forest isn't able to recover itself," he said.

"Once we actually know for certain what's happening, it's going to be too late.

"And this is a big thing to be wrong about."