A tiny marsupial — best known for its tendency to copulate to death — has popped its head up in Canberra's north for the first time in four decades.

Researchers discovered three agile antechinuses while conducting a routine check of traps inside the Mulligans Flat Woodland Sanctuary a fortnight ago, suggesting there was a small but self-sufficient population.

The discovery is now rewriting scientific textbooks, but researchers do not believe it is a coincidence that the marsupial has been found in that location.

After ridding the 485-hectare reserve of predators like cats and foxes, ecologists created a perfect environment for vulnerable species to live and breed — even those not spotted for decades.

Senior ACT Government ecologist Dr Murray Evans said the discovery effectively turned back the environmental clock to a time before the introduction of Australia's feral pest species.

"Agile antechinus were once reasonably widespread in the woodlands and forests of the ACT," he said.

"They're still found in the south of the ACT in places like Namadgi National Park and Tidbinbilla Nature Reserve.

"But the last records of the species from the north of the ACT are from the 1970s — and that's despite numerous surveys."

Many more agile antechinuses are believed to reside within the sanctuary than the few captured on film. ( Supplied: Mulligans Flat Woodland Sanctuary )

Dr Evans said the tiny animal was able to slip in and out of the meshed sanctuary fence, but, understandably, seemed to prefer the safe environment inside the sanctuary.

"To have antechinus just turn up out of the blue and make the sanctuary their home, is just wonderful. You couldn't ask for better," he said.

"Our hope is now that they're in a predator-proof sanctuary, that these animals might be able to increase their populations and become firmly established as one of the woodland animals in this environment."

Once-in-a-lifetime sex

At first glance the tiny marsupials could be mistaken for a native mouse or rat, but the antechinus is no rodent — in fact it is more closely related to Tasmanian devils.

And in many ways the species is much more endearing, due to its bizarre mating rituals.

Males of the species have long baffled researchers with their habit of mating to the point of collapse and death — or, as scientists put it, semelparity.

A male agile antechinus mates to the point of death. ( Supplied: Mulligans Flat Woodland Sanctuary )

At first it was thought to be an altruistic genetic trait to allow surviving females access to scarce food resources.

But it is now believed to be a way of passing on the most *ahem* 'energetic' breeding genes.

"At the end of the breeding season, all males die, so that's about the end of August in this part of the world," Dr Evans said.

"That leaves only females in the population, and the only males present are the unborn males being carried by the pregnant females."

He said females themselves usually died soon after their young were weaned, at about 18 months of age.

Sanctuary about to triple in size

Eastern quolls also call Mulligans Flat home. ( ABC News: James Fettes )

The good news for the agile antechinus — as well as the eastern bettongs and eastern quolls that share their sanctuary home — is that Mulligans Flat is about to triple in size to about 1,200 hectares.

Within a month, construction will be completed on a massive new section of fence behind the growing suburb of Throsby.

But it will still take several years of intensive pest control to eradicate all the rabbits and foxes still living there.

Only then can the joining fence be snipped, and the vulnerable species be released into their new home.

Mulligans Flat Sanctuary manager Dr Will Batson said building fences was not ideal to protect endangered animals, but was the best option ecologists had.

"It's the reality of the world," Dr Batson said.

"These sanctuaries really buy us some time, so we can establish populations that we can then use in the future.

"It also allows us to learn better and develop new techniques that we can use outside those fenced reserves."