Platypuses have been relocated to the Royal National Park in Sydney, after they disappeared from the park's waterways about 50 years ago.
The iconic Australian animal is believed to have disappeared from the national park after a major chemical spill on the Princes Highway in the 1970s, but numbers may already have been in decline.
A joint project by the University of New South Wales, NSW National Parks and Wildlife Service and the World Wildlife Fund has reintroduced five females to the Hacking River, with a group of males to follow next week.
The platypuses were taken from the Bombala and Dalgety regions, and have been kept at Taronga Zoo's purpose-built platypus refuge while they waited to be taken to their new home.
Rob Brewster, from the World Wildlife Fund said separating the sexes initially was key to their early survival.
"We've put females in a week to ten days before the males go in, just to let those girls settle in without those males who are a bit bolder, a bit boisterous," he said.
"Hopefully, those females have found that little niche in their new environment and they can settle in together from there on."
Mr Brewster has been working for several years on making the national park's ecosystems platypus-friendly again, including improving water quality and controlling pests such as foxes and cats.
He said if the initial reintroduction is successful, the platypus will hopefully multiply.
"We're just looking to see if these platypus survive.
"If they do, then obviously breeding, the establishing of burrows and a next generation is a midterm success indicator.
"And beyond that we want to see these platypus spreading out."
The species is Taronga Zoo's emblem and they are "committed to ensuring it thrives".
"Platypus are the silent victims of climate change," director of the zoo's conservation society Cameron Kerr said.
"While their elusive behaviour keeps them from view, under the surface they are particularly susceptible to drought and environmental change."
NSW Environment Minister Penny Sharpe said the relocation was one step to ensuring platypus survived a changing climate.
"Royal National Park is Australia's oldest national park and I am pleased this historic reintroduction will help re-establish a sanctuary for this iconic species," she said.
The platypuses have received veterinary health checks and have been fitted with transmitters, so they can be closely monitored in their new environment.
