This was evidence of breeding by two of the 10 mature platypi "translocated" into the Royal, meaning they had taken to their new surroundings less than a year after being reintroduced there.

But environmentalists warned this was no reason to "go soft" on damage to the waterways from coal mining nearby.

The little female platypus was in excellent condition, weighing 850 grams and estimated to be about six months old, Environment Minister Penny Sharpe said.

Her age aligned with being hatched near the beginning of the platypus breeding season, which runs from October to March, and meant she probably emerged from the burrow about a month ago.

Platypus eggs are incubated under their mother's tail for about 10 days before being suckled for 3-4 months in the 30cm-wide nesting burrow which can be up to 30m long, packed with vegetation.

The young puggle will be named by the local Indigenous community, Ms Sharpe said.

The idiosyncratic monotremes had not been sighted in the Royal National Park for 50 years, their departure believed to have been caused by a chemical spill on the Princes Hwy which polluted the creeks which serve as platypus habitat.

Scientists celebrate

Project leader Dr Gilad Bino from the University of NSW was overjoyed.

"Finding the juvenile platypus is a clear sign the reintroduced population is not just surviving but thriving, adapting well to their environment, and contributing to the genetic diversity and resilience of this iconic species," he said.

It has been a high-profile project to relocate the duck-billed critters into the Royal, with scientists, politicians, researchers, Sydney media and others taken to a site near the Hacking River for part of a detailed survey.

Ms Sharpe took to the tinny to help lay nets on the first night of the survey to catch a platypus. An adult named Delphi was found on the first night and on the second night, the juvenile, she said.

"This is a wonderful discovery by our dedicated scientists and researchers, who have spent their nights patiently monitoring the Hacking River for signs these platypuses have survived in their new home," she said.

"Not only are they surviving, but the discovery of a juvenile platypus shows they are thriving, which means the Royal National Park is in great shape. Invasive species are being dealt with and water quality is good."

'Greenwash' warning

But some environmentalists are not sold on the health of the waterways in the Royal, and question Ms Sharpe's claim that the juvenile is "testament to the health of the park".

The platypus translocation has coincided with penalties and prosecution for coal miner Peabody, which allowed coal wash to spill into Camp Gully Creek, feeding into the Hacking River. Coal material was found all the way to the Audley Weir, past where the platypuses were found.

Some have raised concerns the platypus progress would used to "greenwash" the problems caused when coal mining is allowed so close to the national park.

Brian Everingham from the National Parks Association (NPA) southern Sydney branch said his group was concerned about "about attempts by politicians to claim, somehow, that this means that the water quality within the Hacking is perfectly OK and that coal pollution from recent events in Camp Creek that impact the length of the Hacking is not of concern".

"NPA members have walked many river systems across the state and can vouch that platypus are able to survive in polluted waters," he said.

"Their presence is no excuse to go soft on pollution events such as the many breaches from the Peabody mine."

Last week the miner pleaded guilty to two charges of polluting the waterway brought by the Environment Protection Authority, and one related to failing to maintain its facilities.

The EPA had originally brought five charges and would not specify if or what charges had been dropped when asked by the Mercury, saying this was because the matter was still in progress.

The EPA has fined Peabody multiple times over the runoff and tightened the conditions on its licence, which allows discharge of treated water from the mine into Camp Gully Creek.