Numbers of feral horses in Kosciuszko National Park have exploded in the past few years, a new aerial survey has shown.
A report to be released on Monday shows the numbers of feral horses roaming freely across the landscape in the national park sits at about 25,000.
This is more than double the number of feral horses in the surveyed areas five years ago.
The estimated population in 2014 was 9180 horses, it is now estimated at 25,318, a population increase of 23 per cent each year.
The NSW government was criticised by scientists and environmental groups when, in 2018, it granted the horses heritage status which protected them from culling.
The latest report of the Australian Alps Feral Horse Aerial Survey, which has been conducted five times since 2001, will be presented by NSW National Parks and Wildlife Service on Monday.
An area of more than 7000 square kilometres was surveyed in three distinct locations known to have feral horses.
With the Kosciuszko National Park bordering the Namadgi National Park at the ACT border, ACT Environment Minister Mick Gentleman said the dramatic population increase meant there was an increased threat to the territory's ecosystems.
"I recently toured impacted areas to survey the damage and eroded waterways these heavy hoofed feral animals cause," Mr Gentleman said.
"I have raised and will continue to press this issue with my state and federal government colleagues.
"We need to conserve our water quality and protect critically endangered animals like the northern corroboree frog, which lives in the moist alpine bogs of the ACT high country."
A review of the ACT's feral horse management was recently concluded, Mr Gentleman advised, and findings including the new population figures would be implemented in 2020.
The ecosystem has been altered, has been damaged. Creeks and tributaries are no longer running.ACT Parks and Conservation manager Brett McNamara
The review found the management plan had effectively prevented feral horses from establishing within the ACT but acknowledged the NSW population increase posed significant threat.
ACT Parks and Conservation manager Brett McNamara recalled recently travelling by helicopter to Tantangara near the ACT border and noted the obvious difference between the ACT and NSW.
He said it was "crystal clear" the horses, and also feral deer, had made a significant impact on the areas of "sub-alpine sphagnum mossy bog" which releases water into the ACT catchment areas.
"The ecosystem has been altered, has been damaged. Creeks and tributaries are no longer running. They're not running because the sphagnum is not there to slowly release that water into the ecosystem," Mr McNamara said.
"The reason is it's been trampled and over-grazed."
He said water released from sphagnum moss provided up to 80 per cent of Canberra's drinking water. Water from the Australian Alps also contributes more than 30 per cent of inflows into the Murray-Darling system.
It was crucial, Mr McNamara said, to protect that water source from animals that don't recognise the ACT-NSW border.