Deep in a valley in the ACT's Namadgi National Park, ecologists are puzzling over how a population of goannas are surviving against the odds.

Far from lurking by the BBQ at a summer campground, these reptiles are brushing through snow grass in sometimes freezing conditions.

"They're just not reptile kind of places, particularly not goanna places," researcher Don Fletcher said.

But, according to Mr Fletcher, the even bigger question for scientists was how the lizards were even able to reproduce without a critical part of the landscape.

A few years ago, a population of Rosenberg's goannas was discovered in the Naas Valley near the southern-most tip of the ACT border, much to the confusion of scientists.

The Rosenberg's goanna is the most southerly species of goanna on mainland Australia, usually found in open forests and woodland.

So Mr Fletcher said it was a surprise to find them in a subalpine environment, which could occasionally be covered with snow.

"This location may well be the coldest of the coldest for Rosenberg's goannas," Mr Fletcher said.

But he said, more puzzling still, was that the national park was also missing a particular kind of termite mound needed for the goannas to reproduce.

Goannas use termite mounds like incubators for their eggs. Hatchlings then tunnel up and out after eight months. ( Supplied )

"The most interesting thing about their reproduction is that, in common with some other goannas, the female tunnels into a termite mound and lays her eggs there," he said.

"The termites seal it all over, so you can't tell that there's eggs inside, and eight months later they hatch."

He said the hatchlings then tunnelled out to forage for food in the surrounding area before returning to the termite mound for a final farewell.

"They go back home and eat the termites to say thanks to their hosts before eventually making their way out into the world," Mr Fletcher said.

He said, without the requisite termite mounds, ecologists were left unsure where the goannas at Namadgi had been laying their eggs and how the hatchlings were surviving.

Much still unknown about goannas as populations decline

Goanna numbers are declining across mainland Australia, in part due to predators like foxes and dingoes.

But Mr Fletcher said their own importance as predators was sometimes underestimated.

He said giant goannas — estimated to be 7-metres long and weighing 650 kilograms — once lived in eastern Australia.

"It would have been a terrifying creature to encounter in the landscape, because it would have eaten people and you would have wanted to kill it off," he said.

"Indeed it did disappear not long after the first humans arrived."

Now, with the help of University of Canberra students, more is being discovered about their (much smaller) cousin, the Rosenberg's goanna.

An adult Rosenberg's Goanna, wearing a tracker behind its hind legs, in Namadgi National Park. ( ABC News: Rachel Hayter )

"They all have unique faces," environmental science student Lisa Jokinen said.

"So when we do manage to locate them or capture them, we're able to identify them through facial features."

Ms Jokinen said most of the time the lizards were dark, with white colouring across their noses that almost looked like teeth.

"They also have what appears to be a long division symbol just in front of their eye, and we can tell the goannas apart by how white or how dark the features are on their faces," she said.

Mr Fletcher said the tagging and tracking work was important to determine how the goannas sustained themselves in an unusual environment — something that could help to conserve goanna numbers across Australia.

"Here in the ACT, in this valley, we've got an exceptionally high abundance of them. So why is that? Something's going right for them here. What is it? And how can we apply that to the other reserves?" he said.