But the laughter has stopped, with the native Australian animal now feared to be a vulnerable species as numbers dwindle.

Research from the University of NSW has prompted scientists to nominate the platypus for threatened species listing both nationally and in NSW.

Within Australia's threatened species categories, a "vulnerable" listing is one step under "endangered".

The Australian Conservation Foundation, WWF-Australia and Humane Society International Australia are backing the nomination.

Federal Environment Minister Sussan Ley said the nomination could be assessed as early as next year.

The researchers combed through all available data on platypus observations, finding evidence of past and projected declines in platypus populations to support the animal's listing as vulnerable both on the Australian and international list.

The study found observations were down 32 per cent in NSW, 27 per cent in Queensland and seven per cent across Victoria.

There have been reductions of 18 to 65 per cent in some Melbourne catchments since 1995.

"Given that the causes for these declines have not ceased, will likely continue, and are not reversible in the foreseeable future, the proposed assessment of the platypus is vulnerable," the report says.

"Continued population reduction of platypuses is also projected in the future, based on threats and the impacts of climate change on rivers."

Drought, bushfires, wild predators and illegal fishing nets are some of the main threats to platypus numbers.

In Victoria, 56 per cent of 186 platypus deaths were caused by illegal nets or enclosed traps, which have since been banned in the state but are still available in NSW and Queensland.

"The nature of platypus foraging also makes them particularly susceptible to entanglement around their neck and torso by plastic, fishing line, and rubber bands," the report says.

The semi-aquatic creature is listed as endangered in South Australia and was recently nominated to be listed as vulnerable in Victoria.

Researchers have previously found the platypus was the first animal to have evolved from reptile to mammal and has characteristics of both.

When the first stuffed specimens arrived in Europe at the end of the 18th century, biologists thought they were looking at a taxidermist's hoax of the body of a beaver and the snout of a giant duck stitched together.

If the nomination complies with legal requirements it will be referred to the Threatened Species Scientific Committee for consideration, with the federal environment minister to make the listing decision.

If listed, conservation advice is prepared and a recovery plan could be created.

Flora and fauna listed as threatened under national laws can be put under various categories, such as extinct, extinct in the wild, critically endangered and vulnerable.

Australian Associated Press