Woylie numbers in a sanctuary near Manjimup have quadrupled in two years.
The critically endangered animals have flourished at Perup Sanctuary since the Department of Environment and Conservation released 41 woylies into the 420ha predator-free enclosure in December 2010.
A survey in April found 111 woylies trapped in the sanctuary and trapping in November captured 161 individuals, with all traps full every night.
One female woylie was recorded with twin pouch young in both the April and November surveys, adding to only three previous recorded incidents of twin pouch young in the area since the 1970s.
Woylies were once widespread across southern Australia but were reduced to three small areas by the 1970s.
The species recovered in the 1990s after fox control measures and the animal was taken off threatened species lists.
But their numbers in the wild have plummeted in recent years, with an estimated 90 per cent decline between 2005 and 2007.