More than one-third of the population is believed to have been wiped out in the north of the island state, and there are reports that the disease has now spread to southern areas. It is almost always fatal, causing ulcers that turn into gaping wounds.

The shy, solitary platypus inhabits the waterways of Tasmania and the eastern Australian mainland. The same fungus is found on the mainland, where it kills amphibians, particularly Queensland's green tree frogs. But it does not affect platypus there.

Niall Stewart, a research fellow at the University of Tasmania, believes that the tiny frogs may have carried the fungus into Tasmania in bunches of Queensland bananas.

"Platypus on the mainland have evolved with the fungus, and so they're immune, " he said.

"But the poor platypus here haven't seen it before."

The island is a haven for platypus, thanks to its abundant waterways - wild rivers and pristine streams, as well as lakes and waterholes.

But Dr Stewart, who has carried out extensive field work, believes that 35% of them are falling victim to the disease.

"It's pretty bad, " he said. "It's like when the Europeans introduced smallpox into South America and it killed millions of Indians."

Stewart said nothing was being done to combat the disease. He had repeatedly failed to secure research funding. The problem has been overshadowed by a rare cancer that has killed half of the wild population of another native animal, the Tasmanian devil, and threatens the species' survival.

The ulcers, which appear on the platypus's broad tail or hips, grow to up to 10cm in diameter. Death is usually caused by secondary bacterial infections, or from depletion of body fat, most of which is stored in the tail. The wounds also prevent the platypus from thermo-regulating in cold water. In summer they get infested with maggots.

It is not known how the disease is transmitted - possibly by ticks, or by males fighting, or via burrows, which are occupied by one platypus after another.

Stewart said it was feasible that mud containing fungal spores was being carried into new areas on hikers' boots or four-wheel drive vehicles. If the disease reached some of Tasmania's small offshore islands, he said, it could cause local extinctions.

Asked if the Tasmanian platypus could develop immunity, he replied: "Possibly, in a few hundred thousand years. The problem is that mature animals with ulcers are still capable of breeding, so they're producing more susceptible animals. It would take a long time for natural selection to sort it out."

The platypus is one of only three monotremes - egg-laying mammals - in the world. The others are Australia's two species of echidna, or spiny anteater.