A mysterious disease is threatening the survival of a New South Wales turtle species, wiping out most of its population within two months and baffling scientists who say they have never seen anything like it.

The Bellinger River snapping turtle had been thriving in a near-pristine stretch of waterway on the state's mid-north coast, but has suddenly become endangered.

In a race against time, authorities have established the state's first wildlife biosecurity team to prevent the disease from spreading to nearby waterways.

Seventeen of the turtles are being cared for in special tanks at a quarantine facility at the University of Western Sydney (UWS).

They could be among the last of their kind left on Earth.

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UWS zoologist Ricky Spencer was involved in their collection from the wild before their winter hibernation.

"Their whole range could be affected [by disease] by now," he said.

"We basically had to go in before winter and recover what we could.

"And if their whole range is affected, then there may be no other turtles out there."

The problem was first detected by chance in February, when canoeists discovered dead turtles in the river.

Since then, it has dramatically escalated. The remains of more than 400 turtles have now been found, and Dr Spencer said many more had probably been killed by the mystery disease.

"We know that it wipes them out very, very quickly.

"Individuals get lesions, particularly around their eye, but throughout their whole body there are these lesions that basically kill them.

"It has a 100 per cent mortality rate."

Seventeen turtles are being cared for in special tanks at a quarantine facility at the University of Western Sydney. ( Supplied: Sally Tsoutas/University of Western Sydney )

Dr Spencer said the cause of the outbreak was "completely unknown".

"It doesn't seem to be one mystery virus or disease that they can actually find," he said.

Scientists are unable to explain the mystery disease affecting the turtle population. ( ABC News: Antonette Collins )

Rescuing the turtles has been a major operation involving state authorities, Sydney's Taronga Zoo and a team of wildlife experts.

Lawrence Orel, from the NSW Office of Environment and Heritage, said there had been a number of efforts to determine the cause and extent of the problem.

He said there were concerns for two other unique turtle species in the region, and the rest of the aquatic ecosystem, but the disease appeared to only be affecting the Bellinger River snapping turtle for now," he said.

"Unfortunate as it is for that species, we've not been able to detect any other impact on other species or turtles in the river, or other fauna in the river."