There are fears a critically endangered possum endemic to southern Western Australia could be wiped out within two decades if there is not a serious intervention by government.

Much of the western ringtail possums' habitat has been taken over with development, while it also faces threats from feral predators, forest dieback and bushfire.

Associate Professor for the University of Western Australia's School of Agriculture and Environment, Roberta Bencini, said habitat fragmentation had also lead to genetic issues like inbreeding.

"There is a 92 per cent risk of extinction in the next 20 years unless we do something to save this animal," she said.

Dr Bencini oversaw the 2013 construction of a rope bridge across a Bunbury bypass highway, after the highway flattened a significant portion of the possum's habitat.

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However, motion-activated censor cameras on the bridge only recorded two crossings in 270 nights.

Dr Bencini claims her advice about where to put the bridge was disregarded in favour of a more cost-effective option.

"It would have been better if it had been put in the valley further west because that's where we observed most of the possums — and where we thought it should be in the first place," she said.

Main Roads said the bridge was not a failure, but more a demonstration that the lengthy size and configuration of the bridge has not encouraged use by animals.

The agency said it was working to improve the sparse vegetation around the bridge, which it hoped might encourage the possums to cross.

It was a different story for a second, smaller rope bridge that allowed possums to cross Caves Road near Dunsborough, with 1300 confirmed crossings in the same period.

Dr Bencini said the bridges alone were not enough to foster population growth and without further investment, extinction was certain.

Injured pose big threat to the possums

It was estimated about 200 western ringtail possums are injured or killed every year in the Busselton area, many of which end up with local wildlife group FAWNA.

Hundreds of injured western ringtail possums end up in care each year. ( ABC News: Gian de Poloni )

The group's Tiana Schultz said they were an extremely vulnerable animal.

"We get all injuries and ailments," Ms Schultz said.

"Some days we have possums that have burned feet from walking across barbecues in the evenings, from broken bones from vehicles and bites from cats and dogs.

"A single scratch from a cat will kill a possum within two days."

The Department of Parks and Wildlife produced a comprehensive plan in 2014 aimed at slowing the population decline.

Signs warn drivers to watch out for possums on the road. ( ABC News: Gian de Poloni )

It recommended several actions to link up and improve the quality of the possums' habitat and more effectively control predators.

Dr Bencini was also applying for a grant for a new rope bridge across a canal at Loch Nature Reserve and crowdfunding to raise money to continue field monitoring of the species.

$73 million dollars was being spent this financial year to assist DPAW with the recovery of threatened species.

But the agency could not say exactly how much of that will go towards the possums.

"All critically endangered species — in fact, all threatened species — are a priority for the department and we work to recover all of them," DPAW assistant director for conservation Fran Stanley said.

Ms Schultz suggested money would be well spent investing in a breeding program or conservation hub targeted at the possums.

"Yes, we're caring for them but we're not helping them breed and helping their species as such," she said.

Environment Minister Stephen Dawson said his Department was continuing to manage threats to the possums and their habitats such as feral cats and foxes.