The iconically-strange platypus of Australia is coming home to a national park near Sydney 50 years after their mysterious disappearance.

Whether from a chemical tanker spill on the nearby Princes Highway or over-predation from foxes or cats, conservationists aren't quite sure when and why the strange egg-laying aquatic mammal vanished from Royal National Park's rivers, but they're back now.

A joint project from the University of New South Wales, National Parks and Wildlife Service, and WWF, conservationists released 5, (or perhaps 6) females last week into Hacking River as the first stage of the reintroduction.

Kept at Taronga Zoo's special platypus refuge, the 5 females came from the Bombala and Dalgety regions.

"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," said Rob Brewster, an Aussie conservationist with the WWF. "Hopefully, those females have found that little niche in their new environment and they can settle in together from there on."

Brewster said the collaboration wants to see burrows and territorial establishment among the river systems of the 58-square-mile national park from the females before they release the males.

Beyond ensuring that the water quality in the catchment areas outside Royal National Park is of the highest level, threat management teams are also working to ensure the platypuses are not attacked by foxes or feral cats.

"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," said NSW Environment Minister Penny Sharpe.

Short-term indicators for success, Brewster explains, are principally any sort of breeding activity.

WATCH some of the reintroduction footage below… 

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