Aussie Ark has returned five platypus to the wild that were in distress in the Hunter River catchment during the bushfire crisis and drought.
The native mammals were "severely malnourished" and their waterproof fur was showing signs of "extreme distress".
They were found at Moonan Brook in the Barrington Tops in muddy pools of water no bigger than backyard swimming pools and were "essentially swimming in their own faecal matter".
They had little to no food available and rising water temperatures were threatening their lives.
Aussie Ark President Tim Faulkner said the platypus were "so emaciated and near death, they had to be brought into care".
"Most weighed between 300 and 400 grams. They now weigh a kilo and that's normal weight," Mr Faulkner said.
Having been nursed back to health, the creatures were returned to the same suitably recovered river system.
But while five creatures were saved, other platypus perished.
Meanwhile, UNSW Sydney research released on Monday found the platypus is in peril and should be recognised as a threatened species. The research, commissioned by Australian Conservation Foundation, found the decline in platypus observations was most severe in NSW (32 per cent) and Queensland (27 per cent).
"Platypuses are to rivers what koalas are to forests," said Dr Stuart Blanch, of WWF-Australia.
"These days a sighting of just a few platypuses is often associated with a healthy population, but historical records suggest todays numbers are only a fraction of what they once were. This alarming decline is the wake-up call we need to better protect our rivers and creeks."
New dams, the over-extraction of water from rivers, land clearing, attacks by foxes and dogs, pollution and suburban sprawl are the main factors driving the decline.
Mr Faulkner said: "Platypus have been on Earth for nearly 200 million years unchanged. Events like this are changing them now. Let's not let them disappear in our lifetime."
Professor Richard Kingsford, director of the Centre for Ecosystem Science at UNSW, said protecting the platypus and the rivers it relies on "must be a national priority for one of the world's most iconic animals".
"There is a real concern that platypus populations will disappear from some of our rivers without returning, if rivers keep degrading with droughts and dams," Professor Kingsford said.
"We have a national and international responsibility to look after this unique animal and the signs are not good. Platypus are declining and we need to do something about threats to the species before it is too late."
Dr Paul Sinclair, campaigns director at the Australian Conservation Foundation, said national environmental laws should be much stronger, but "listing the platypus as a threatened species is a critical first step towards conserving this iconic Australian species and putting it on a path to recovery".