Poor water management of the Murray-Darling Basin has led to an alarming decrease in platypus numbers, researchers say.

UNSW scientists say declines in Australia's platypus population appear to have been underestimated. They suggest numbers may have actually halved since Europeans arrived.

Professor Richard Kingsford has been studying the platypus for the past four years, and has found “that there's been this huge decline in platypus numbers over the last 100 to 200 years and we're not really dealing with that”.

The famously bizarre animal is disappearing. There have been no official observations in the last 10 years in half of the catchments of the Murray-Darling basin.

The UNSW research project analysed 258 years’ worth of data on platypus distribution and abundance, comparing it with recent data from three years of surveys.

Current population estimates ranging from 30,000 to 300,000, researcher Tahneal Hawke says.

“In the western-flowing rivers, platypus were once abundant in the hundreds, but now again it's rare to see platypus on those rivers,” Ms Hawke said.

Researchers say the animal has never recovered after being hunted between the 18th and 19th centuries.

“There was one record of a single furrier selling up to 29,000 platypus skins before the First World War, so we think that has caused long-term impacts across their range,” Ms Hawke said.

Professor Kingsford says much more needs to be done.

“It's important that the Murray-Darling Basin Authority work with the states to highlight the importance of ensuring there is enough water in the dry times, like now, for platypus to survive,” he said.

Murray-Darling Basin Authority spokesman Carl Binning said it is not surprising news.

“The Murray-Darling Basin is our largest river system — we've recognised that system has been over-allocated and that's what the authority and the basin plan is all about,” he said.

“Platypus are really interesting creatures — they're unique and they really like pristine habitat so I wouldn't expect that we're going to be able to recover platypus populations across the basin.

“But in some of our rivers that are in better shape there are still platypus and we really need to look after them.”