A small carnivorous marsupial long thought extinct in New South Wales has been discovered in the far north-west corner of the state.

The crest-tailed mulgara was known only from fossilised bone fragments in the state for more than century.

But a team from the University of New South Wales (UNSW) made the unexpected discovery during a recent project in Sturt National Park near Tibooburra.

UNSW scientist Reece Pedler said he could hardly believe it when he saw the animal.

"There was a lot of excitement when we looked in to a trap and there was a mulgara," he said.

"I had to pinch myself. I certainly wasn't expecting to find one.

"Twenty-five years ago, if you wanted to find them you'd have to go to the Simpson Desert.

"But over the last 20 years, since rabbit calicivirus has brought some real changes to arid ecosystems, this little critter's been making a massive comeback."

The devil's cousin

The small carnivorous marsupial is closely related to the Tasmanian devil, and Dr Pedler said like the devil, they would eat just about anything.

"They are non-specialist little carnivores. They scavenge meat and insects," he said.

"I've seen them eating mice and small birds, even roadkill rabbit."

Unlike their larger, more well-known cousin, the mulgara had few defences against introduced species.

UNSW scientist Rebecca West said Australia's small mammals barely had a chance against dogs, cats and foxes.

"A lot of these species were in that weight range of animals that are really susceptible to predation and they hadn't evolved with those predators," she said.

"These new beasts rocked up on the block with different hunting skills to the ones they were used to dealing with, and they just couldn't cope."

The crest-tailed mulgara discovered in a National Parks trap at Sturt National Park. ( Supplied: UNSW )

New hope for program

The discovery of the mulgara was made all the more significant on the eve of the launch of the Wild Deserts program.

The project hopes to eradicate cats, foxes and rabbits from two 20-square-kilometre fenced enclosures in Sturt National Park, before introducing seven different locally extinct mammals, including the mulgara.

Dr Pedler is a Wild Deserts project co-ordinator, and said the timing of the mulgara's return was perfect.

"Given this work we're about to undertake removing cats, rabbits and foxes from a really large area, the timing couldn't be more perfect," he said.

"It provides a real opportunity for the mulgara to do really well at this site, as well as a whole range of native species. "

It is hoped the construction of the enclosures will begin in 2018, and the first of species to be introduced to the area, the greater bilby, will be released in 2019.