Environmentalists are considering a legal challenge to stop a proposed 800-bed development at Perisher in Kosciuszko National Park.

The Colong Foundation for Wilderness said it had visual evidence that Perisher's owner Vail resorts did not start work on the seven-building development by October 1 this year and, as a result, its concept approval had subsequently lapsed.

It has engaged the Environmental Defenders Office (EDO) and written to National Parks seeking clarification of whether works had started.

Director Keith Muir said they had photos taken in mid-October showing the developer had not started work on an interpretive trail and seating that would have signalled the start of construction.

The Colong Foundation for Wilderness says this photo taken on October 20 shows work had not started on time on a chair and interpretive trail at Perisher. ( Supplied: Colong Foundation for Wilderness )

The long-awaited development had a concept plan approved as far back as 2006.

Vail became its owners when they bought Perisher from James Packer and Transfield earlier this year for $176 million.

There have been ongoing concerns about the impact of commercial interests in the National Park and potential impact of an extra 800 overnight guests in peak season.

Breach of conditions

The ABC has learned the on-site sewage treatment plant breached its licence conditions in eight areas during the busy winter season this year.

This meant water with above acceptable limits of nitrogen, phosphorous, suspended solids and biochemical oxygen demand was released into the park's streams and creeks.

Park operators also moved an endangered sphagnum bog in 2014 to make way for the $4 million Freedom Chairlift for Perisher.

Mr Muir said ski resorts had been moving protected plants and granite rock tors to make way for ski runs for many years.

"So the bigger the resort the more slope grooming, the more removal of habitat for endangered mountain pygmy possums, burramys, the vegetation that they live on," Mr Muir said.

"That's a very significant impact on a threatened species."

This fragmented the park and created natural boundaries for native flora and fauna.

"The Colong Foundation is reviewing the legal situation in regard to this concept plan approval. It's not clear to us that this approval is legal and were getting the EDO in NSW to look at it for us," Mr Muir said.

"We oppose the development, it's a massive development, it will turn what is a small village into a town in a national park."

Got a confidential news tip? Email ABC Investigations at investigations@abc.net.au For more sensitive information: Text message using the Signal phone app +61 436 369 072 No system is 100 per cent secure, but the Signal app uses end-to-end encryption and can protect your identity. Please read the terms and conditions.

These are the plans the Colong Foundation for Wilderness claim have not been started. ( Supplied: The Colong Foundation for Wilderness )

'Actual impact is minimal', Department says

In response, NSW Planning and Environment said a construction certificate was issued in June and department staff inspected the site on November 1, and verified that work had started.

"The department is therefore of the view that the Concept Plan remain operative," it said in a letter to Mr Muir.

Robert Quirk from the National Parks and Wildlife Service said while the concept plans were approved, any new buildings would have to have a separate development application.

"The approval, is approval as a concept only and a development application would need to be done and all those factors would need to be built-in to the assessment," he said.

He said some of the sewage overflows happened during storm events.

"There is no doubt it tested the system," he said.

"The only advantages of those major storms is the dilation that occurs, the amount of water moving through the system, the actual impact is very minimal.

"The sewage system has very tight licensing requirements, any time there is a breach it has to be reported. They all get reported back to the EPA, they get assessed, they get cleaned up."

There have been ongoing concerns about the potential impact of an extra 800 overnight guests in peak season. ( ABC News: Alison Branley )

Efforts to lessen environmental impact

The NSW Environmental Protection Agency said in a statement it was aware the plant had breached its limits.

"Water quality monitoring, both up and downstream, found that the ambient water quality of Perisher Creek was not affected by any of the exceedances, and there was no measurable environmental impact," it said.

A spokeswoman said EPA officers were continuing to monitor activities at the site and National Parks had made improvement including recalibrating equipment and introducing extra microbes into the system.

An image of Perisher Village from the ABC's visit in December. ( ABC News: Alison Branley )

"We expect Perisher Sewage Treatment Plant to actively work at improving their environmental performance overall," she said.

Vail Resorts did not respond to questions about the proposed development on the sewage treatment plant.

In a statement Perisher Ski Resort chief operating officer Peter Brulisauer said Vail Resorts' environmental efforts at Perisher were focused on protecting the natural habitats and wildlife in Kosciuszko National Park.

"Towards this end, we have closed off parts of the resort for the protection of the mountain pygmy possum," he said.

"We have planted over 7,000 native shrubs, trees and grasses throughout the resort since 2013.

"In addition, the construction of animal crossings to safely guide wildlife across ski slopes has resulted in the protection of the Mountain Pygmy-possum and other small mammals.

"Perisher Ski Resort [is] committed to caring for and preserving the natural environments in which we operate, including conserving natural resources, lessening our footprint and engaging our employees, communities and guests as we strive to protect these iconic locations."