NSW Deputy Premier John Barilaro says the promised reduction would first require a tender process for a population count and management plan.

Before the election, the Nationals leader said he would look to reduce Kosciuszko National Park's population of brumbies with non-lethal methods by 50 per cent "immediately" if elected.

Mr Barilaro is the member for Monaro, which ranges from north of Queanbeyan down to the Snowy Mountains.

"Before we can determine how many brumbies are to remain in [the park], there needs to be a proper count," Mr Barilaro said.

"We first need to know how many there currently are."

He said a draft management plan would need to be drawn up, and while he wouldn't pre-empt what that plan would suggest, he assumed it would be a 50 per cent reduction.

Conservationists fear the estimated 6000-strong feral horse population in the park is leading to widespread environmental destruction and loss of habitat for native species.

The horses favour the delicate wetlands in Australia's alpine regions, home to sphagnum bogs, which house rare species like the corroboree frog. The horses also cause erosion along riverbanks, damaging the flow of the rivers.

University of Canberra ecologist Mark Lintermans recently suggested fencing off a three-kilometre stretch of river in the park, which was the only known home to a native fish and which is being destroyed by brumbies.

The ACT Environment Minister and park rangers have their own concerns about how an unchecked population may soon move into the ACT and destroy local drinking-water catchments.

The NSW government granted heritage recognition to feral horses last year under the so-called "Brumby Bill", allowing only non-lethal methods to control horse numbers, including rehoming.

At the same time it threw out a 2016 draft management plan which suggested reducing Kosciuszko's 6000 feral horses down to only 600 using methods including aerial and ground shooting over 20 years.

The Brumby Bill was spearheaded by Mr Barilaro in 2018, who then made comments before the state election that he would look to reduce the horse population by 50 per cent "immediately".

University of New South Wales Canberra scientist David Paull has been using drones since 2013 to create 3D maps of riverbanks in the park.

"Where the horses were present they were causing ongoing erosion," Dr Paull said.

Dr Paull mapped one side of a 15-metre section of the Ingeegoodbee River in the park's south-west, close to the Victorian border.

There the riverbanks have been eroding at a rate of 70 kilograms of soil per metre, per year.

"When you think about it, it's a lot," Dr Paull said.

He warned this disturbed soil would settle in the river, muddying the aquatic habitat, then be transported downstream.

But Dr Paull said there were parts were horses were present that weren't showing such severe erosion.

He said - depending on the section of the park - it was due to higher-altitude areas being snowed under in winter or tussocky areas making it dangerous for horses trying to reach rivers.

"We see plenty of lame horses out there when we're doing fieldwork," Dr Paull said.

"They're probably hurting themselves by traversing areas that are not suitable for horses."

But he said eventually horses would be able to access riverbanks in less-eroded parkland, eventually accelerating erosion across the rivers.

Deakin University ecologist Don Driscoll said an immediate reduction would have been impossible using non-lethal methods.

"That's impossible considering the methods they've used," Professor Driscoll said.

He said aerial culling would be the most humane and efficient way to rid Kosciuszko of a pest that has proven "environmentally disastrous" for the park.

"The thing that people really must do is think beyond the bullet," he said.

He said rehoming feral horses would put them under more stress due to corralling, capturing and transporting the horses, and it was also more costly than culling.

Professor Driscoll also said while brumby advocates were concerned about the horses dying, he said native species were losing their habitats because of the destruction caused by horses and were potentially dying painful deaths in the process.