The missing marsupials are not vulnerable babies, just out of the pouch, but healthy sub-adults.

"We have wondered if the missing sub-adults are succumbing to predation or dispersing from the refuge-rich habitat of the core colony," said Juliet Dingle, contractor on the Kangaroo Valley Brush-tailed Rock-wallaby Recovery Program.

READ MORE:

To discover the fate of the animals, Friend of the Brush-tailed Rock-wallaby, NSW National Parks and Wildlife and Saving our Species decided to trap, radio-collar and track some sub-adults.

The Friends of the Brush-tailed Rock-wallaby group secured a grant through the Foundation for National Parks and Wildlife to purchase a radio-tracking antenna and receiver, hi-tech 'GPS' collars and a PinPoint Commander unit and reader capable of mapping an animal's movement over a year.

"The new collars have the added benefit of a time-release function, which means the rock-wallabies should not have to be trapped again to remove the collars," Ms Dingle explained.

Male Brush-tailed Rock-wallabies reach sexual maturity from 24 months and this is often when they are ushered out of their colony by the alpha males.

"Until about 50 years ago there were neighbouring colonies and this dispersal behaviour by males facilitated important gene flow within the species," Ms Dingle said.

There are now only two known colonies in the Kangaroo Valley area, however.

At the start of September two four-kilogram 20-month-old males, Magic and Ruben, were trapped at the River colony and fitted with the light-weight VHF radio-collars.

Experienced NPWS and Saving our Species staff helped with the fieldwork and provided the soft-sided traps, scientific licence and ethics approval.

The Friends group are funding the contract for the trapping and radio-tracking of the sub adult rock-wallabies and hope to involve a committed Friends volunteer after she finishes her HSC.

Despite the excellent technology involved the project faces challenges, due to the remote location of the animals. So far, however, it has been successful.

The Friends group extended their thanks to the Foundation for National Parks and Wildlife for the opportunity to learn from the adventures of young Ruben and Magic in Kangaroo Valley over the next year.

"The more we understand about the local population of endangered Brush-tailed Rock-wallabies, the better chance we have of securing its future," Ms Dingle said.