A New South Wales north coast turtle species is now considered to be on the brink of extinction after a mystery virus all but wiped it out 12 months ago.

The NSW Office of Environment and Heritage said the source of the virus had still not been pinpointed.

Multi-agency team surveying for turtles in the Bellinger River, in late 2015. ( NSW Scientific Committee )

In February last year hundreds of dead and dying turtles washed up on the banks of the Bellinger River, on the NSW mid north coast.

Office of Environment and Heritage regional manager Don Arnold said there were very few Bellinger River Snapping Turtles left in the waterway.

"The population has been decimated to only a handful of animals now," Mr Arnold said.

"When our staff were in the field just recently they identified possibly up to 60 animals. That's down from a population of somewhere between 1,500 and 3,000 animals.

"One of the most baffling things about this particular mortality event is how rapidly it's happened."

Mr Arnold said the NSW Scientific Committee had now listed the Bellinger River Snapping Turtle as critically endangered.

"I guess it's an identification that this species is at high risk of going extinct in the near future," Mr Arnold said.

"It demonstrates that they have gone, as a consequence of the disease outbreak approximately this time last year.

"The mass mortality event has seen the population decimated, to where there is only a handful of animals now."

Bellinger River relatively pristine

Mr Arnold said the virus occurred in a more remote section of the Bellinger River, upriver from the town itself.

Bellinger River Snapping Turtle in captive breeding program. ( NSW Scientific Committee )

"This turtle occurs in the upper-middle reaches of the river system," he said.

"Most of the catchment area is national park and it's relatively pristine.

"Of all the river systems on the north coast, the Bellinger River system is probably one with better health, compared with some of the others.

"So it is surprising that it occurs in this particular river system."

Mr Arnold said the turtle was unique to the Bellinger River and that is why it was listed as critically endangered.

He said hope for the species survival now rested with a captive breeding program.

"We're working actively with Taronga Zoo and the University of Western Sydney," Mr Arnold said.

"We still hold 16 healthy turtles that we're hopeful this spring early summer, we may be able to successfully breed.

"The captive breeding program is critical to this species because we suspect the population that's left in the wild is not enough to sustain itself."