Ten of the mammals - a mixture of male and female - will be released in 2022 in a project being led by University of NSW Sydney scientists in collaboration with WWF (World Wildlife Fund) Australia and National Parks and Wildlife Service (NPWS).

The platypuses will be fitted with acoustic tags, which will enable researchers to track their progress and any breeding activity for up to two years.

Read more: The Illawarra animals on NSW's threatened species list

Researchers will survey rivers and streams in Royal National Park and Heathcote National Park to identify suitable habitat and confirm there are no remaining platypuses.

NPWS will provide public viewing infrastructure once the initial population is established. This will consist of specialist boardwalks and viewing platforms designed to have minimal impact on platypus habitat.

UNSW says the last recorded sighting of a platypus in the area was in the 1970s.

They are thought to have been wiped out by a chemical spill on Princes Highway, which washed into park streams.

Dr Gilad Bino, from UNSW's Centre for Ecosystem Science, said the aim was to introduce an initial group of 10 platypuses into the park late next year.

"We'll monitor how they settle in, with the hopes their population will start to grow and reestablish in the area and allow people a place to come and appreciate this unique animal," he said.

"Platypuses are hard to see and are mainly nocturnal, so we have to survey and assess the condition of all the rivers and creeks in the park.

Read more: The lies the lyrebird tells to keep his girl

"This will allow us to verify the absence of platypus and it also means we can check which part of the rivers are suitable for reintroducing new individuals."

Environment Minister Matt Kean said the project would help ensure the platypus' future.

"The platypus is seen nowhere else on the planet and like so many of our other iconic native species its future is uncertain," he said.

"Unfortunately, we have some of the worst extinction rates anywhere in the world and we have to make sure the platypus never makes that list."

For more environment news, click here.

The Illawarra Mercury news app is now officially live on both iOS and Android devices. It is available for download in the Apple Store and Google Play.