But the joey she has suckled as her own doesn't share her distinctive yellow-ringed tail.

The joey that grew inside her pouch is a critically endangered southern brush-tailed rock wallaby.

Fewer than 80 of its kind survive in Australia, with about 40 in captivity.

Twenty-three of those in captivity live in the Tidbinbilla Nature Reserve, south of Canberra.

Tidbinbilla wildlife team leader Dr Jenny Pierson said "cross-fostering" southern brush-tailed rock wallaby joeys with other wallaby species was helping to bring the species back from the brink of extinction.

"One of the limitations [in breeding these wallabies] is the animal has to basically hold its joey in its pouch and lactate for six months," Dr Pierson said.

"If we can use a different wallaby to perform that for her then she can have another baby and we can have more southern brush-tailed rock wallabies."

Cross-fostering is an ethically-challenging conservation technique but one that has enabled this unique species to cling on in pockets of its former ranges.

The joeys are taken from their mothers' teats when they are aged from 22 to 30 days.

They are then attached to the teat of a female yellow-footed rock wallaby, like Lily, while the yellow-footed rock wallaby joey is humanely euthanised.

"Once they're attached to the teat of the other one she basically becomes a surrogate mum," Dr Pierson said.

"They make very good surrogate mums, the young tend to do incredibly well with the yellow-footed rock wallabies. We used to use the tammar wallabies but they actually do a little bit better with the yellow-footed rock wallabies."

Once the southern brush-tailed rock wallaby joeys are old enough to become independent they are moved to the larger enclosures with the other brush-tailed rock wallabies to learn their behaviours.

It has been 20 years since the brush-tailed rock wallaby was declared endangered in the ACT.

Cross-fostering as a conservation technique was first used with the southern brush-tailed rock wallaby and Tidbinbilla was involved in the early trials, Dr Pierson said.

The nature reserve has since bred more than 70 joeys, 28 of which were re-introduced into Victoria's Grampians National Park from 2008-13.

But hampering their recovery as a species are the dwindling cavities of predator-free terrain.

"One of the main threat is fox predation so they originally declined quite a bit due to a combination of threats including the fur trade, fox predation and habitat loss," Dr Pierson said.

The southern brush-tailed rock wallabies at Tidbinbilla are protected by a towering predator-proof fence.

But it's protective mother wallabies like Lily that are fostering hope for this species' future.