The experiment, which has so far cost the territory government about $3.5 million, was intended to "garner information" about the mammals rather than reestablish a wild population, Environment Minister Mick Gentleman said in 2017.

Fox control expert Dr Mike Braysher says it would probably take about 2000 bettongs - nearly nine times the ACT's current captive population - to have a chance at achieving a new population.

"You're getting poor, captive-bred animals and putting them out in the wild to be eaten by foxes and cats," Dr Braysher said.

"That would hardly be considered humane, I would have thought."

Manager of Mulligans Flat Woodland Sanctuary, Dr Will Batson, agreed they would need a lot more animals for a reintroduction to be viable, although just how many was yet to be determined.

"That number has to account for the expected level of mortality. That's why you need to do the set-up work, these trials," he said.

The release program's organisers and funding partners have hailed it a success, despite many of the 67 bettongs having been killed by foxes immediately or shortly after their release.

The purpose of the trial was to test whether eastern bettongs, which have long been extinct on mainland Australia, could survive in an environment with intensive fox control, ACT Parks and Conservation director Daniel Iglesias said.

The longest-surviving bettong died in March 2018 after 18 months in the Lower Cotter. All of them had been reared in captivity at Mulligans Flat or Tidbinbilla Nature Reserve and released at about nine months old.

The program was initially expected to be finished in March 2017.

"The only way to robustly evaluate the hypothesis ... was to test it in the real world and carefully monitor the outcomes," Mr Iglesias said.

Dr Braysher argues there is countless research that has been done on the subject, so the outcome of the release program was obvious.

"If they read any of the papers, they would have known that they were doomed to die," Dr Braysher said.

The program was run by the Australian National University in liaison with the ACT government, commonwealth government, Woodlands and Wetlands Trust and both nature reserves.

The university received about $127,000 in commonwealth government funding to manage the bettong program and other future reintroductions at Mulligans Flat, while the trust contributed to the managing post-doctoral fellow's salary.

The Wetlands and Woodlands Trust was contacted to confirm the amount of the contribution.

Construction of a fence for bettongs in Mulligans Flat set the ACT government back $1.3 million in 2009 and maintenance of the fence has cost $144,000 a year - or about $1.3 million - since.

The Lower Cotter release program cost $600,000 over three and a half years from when the program started in mid-2015 to when the last fox baits and traps had been laid in early 2019.

Of that $600,000, about $525,000 went to employing three people, or the equivalent of one full-time ranger, to manage fox baiting, bettong monitoring, and other activities like weeding.

"It's exposing our staff to the skills and experiences that are required for reintroduction of native animals," Mr Iglesias said.

"One of the [biggest challenges] in Australia today is getting those skills."

Mulligans Flat and Tidbinbilla contributed staff to the program but no cash.

Dr Batson was not ruling out similar experiments and could introduce predators into Mulligans Flat to improve animals' predator sense.

"The findings [of this experiment] ... will bring the return of our extinct native mammals to mainland Australia much closer," an Australian National University spokesman said.