ANU scientists say dead or dying manna gums scattered over 2,000 square kilometres on the Monaro Plains in southern NSW will never recover.

No-one knows if the dieback was due to farming, drought or a lack of traditional burning.

Attention is now being turned to the introduction of another species, and whether that would be successful.

Australian National University Fenner School of Environment and Society associate professor Cris Brack said the affected area in southern New South Wales was so vast people just could not get a sense of its size.

He said it was roughly the size of the ACT, covering land from Cooma to Jindabyne and down to the Snowy River.

The dominant tree species, and an iconic part of the Monaro Plains, is Eucalyptus viminalis, commonly known as ribbon gum or manna gum.

Dr Brack said the area was one of Australia's naturally treeless plains and natural grasslands.

"But there are trees on the tops of hills, ridgelines, and a few scattered trees in those natural grasslands," he said.

'No-one thinks those manna gums are going to survive'

Dr Brack said the trees had a particular importance to the environment and ecosystem.

He described them now as being "skeletons in huge tree graveyards".

"Each tree lost might be even more significant than, say, losing them in a traditional forest," Dr Brack said.

"No-one thinks those manna gums are going to survive. It's just too far gone and we don't think they can survive into the near future.

Map of Monaro Plains dieback area ( Supplied: Cris Brack )

"It looks like they're the ones that have been devastated by [dieback]. Almost all of them are dead. A couple associated species are affected, but nowhere near the extent of the manna gum."

The cause remains a mystery.

"We've managed to say a number of things that isn't [a cause], but we still don't quite know what it is," Dr Brack said.

The associate professor joined researcher Catherine Ross to conduct a field survey to map the entire affected area.

The survey revealed a widespread infestation of an endemic but previously undescribed species of eucalyptus weevil (Gonipterus sp.).

"Very often insects are involved, but they're often involved right at the end, and that's what seems to be the final straw for the eucalypts," Dr Brack said.

"Probably one of the underpinning causes is that trees are getting unusually stressed and can't fight back, but other causes are adding to that."

Dr Brack believes the dieback began around the time of the millennium drought (2001–2009) or perhaps earlier.

However, he said other droughts had not caused dieback, and their research showed no evidence the cause was land management practices or fire history.

In a recent article, the two researchers wrote that manna gums "appeared to be uniformly dead or showing signs of severe dieback regardless of their local environment", and that the ultimate cause seemed to be the infestation of eucalyptus weevil.

Critic says traditional low intensity burning should be tested

The researchers noted dieback was often associated with insect attack, but the underpinning reasons were much more complex, citing a paper on dieback written by Vic Jurskis.

Mr Jurskis told the ABC he believed the cause was clearly a lack of low intensity fire, pointing to his proposition that Australian ecology was based upon 'firestick' burning practices used by Aboriginal people for tens of thousands of years.

He criticised the study for not testing that proposition.

"And now it's nearly too late. What they need to do is get any of the remnant native vegetation ... and introduce frequent mild fire in there and they'll soon see that things improve."

If it is too late for the manna gums, then Dr Brack is looking to the future and the options to replace the lost species.

"If climate change is true... then the Cooma-Monaro area is going to be changing its environment," he said.

"So we have to plant now or find species now that will survive whatever the climate's going to be in 50 years time."

However, he despairs that finding a solution is not a priority for research funding.

"There's no one with a specific mandate to look at this sort of issue."