Wild horses running through Australia's alpine country are booming in numbers under new legal protections handed to them by the New South Wales Government.

Key points: A new survey reveals brumby numbers have grown from 9,187 to 25,318 in five years

A new survey reveals brumby numbers have grown from 9,187 to 25,318 in five years Brumbies are culled in Victoria and the ACT, but last year were handed some protection — prohibiting lethal culling— by the NSW Government

Brumbies are culled in Victoria and the ACT, but last year were handed some protection — prohibiting lethal culling— by the NSW Government The ACT's Environment Minister says the population needs to be managed to reduce damage to the landscape and waterways

Survey data collected by the Australian Alps Liaison Committee shows the horse population has more than doubled in just five years, again raising suggestions they should be culled.

According to the survey, the number of horses has grown from an estimated 9,187 in 2014, to 25,318 in 2019.

That amounts to a growth rate of 23 per cent per annum, despite ongoing drought conditions.

The figures have been labelled "alarming" by the ACT's Environment Minister and will likely reignite the debate over how the horses should be managed.

Why have numbers soared?

The brumbies are currently protected in NSW. ( ABC News: Ginny Stein )

Brumbies are culled in both Victoria and the ACT, but last year were controversially handed a level of legal protection through the NSW Government's so-called 'Brumby Bill'.

It recognised the cultural significance of the horses and prohibited lethal culling; however, it did not prevent reducing the number of horses through non-lethal means.

In response to the figures, NSW Environment Minister Matt Kean said the brumby population did need to be controlled.

"These numbers are unacceptable and unsustainable for our natural environment," he said.

"The NSW Government will work with the community and scientific advisory committees to take steps to reduce the number of horses in the National Park in a humane way."

ACT Environment Minister Mick Gentleman said the horses did not need to be totally eradicated, but it was clear there should be far fewer living in the Australian Alps.

"You can manage the numbers and still have some brumbies in the area," he said.

"But you really need to manage the numbers so they don't do the degradation we're seeing in Kosciuszko National Park."

The Australian Alps Liaison Committee survey revealed an extraordinary boom in horse numbers in the north-eastern corner of Kosciuszko, which takes in the Tantangara Reservoir, sitting north of Adaminaby and bordering Namadgi National Park to the north-east.

Between 2014 and 2019, numbers in that area lifted from an estimated 3,255 to 15,687 — a growth rate that could see horse numbers double nearly every two years.

The detailed survey report suggested the soaring numbers could come down to the relatively open, plains habitat in that area.

It also suggested there had been little effort to reduce the number of horses over the five years.

"There has been little or no management of the feral horse population in North Kosciuszko in the intervening period between surveys, with records showing that only 881 horses were trapped and removed from the area in that five-year period."

Further south, the number of horses has also grown steadily, but not at the same pace.

In the survey area covering Mt Kosciuszko in the north, over the border into Victoria's Alpine National Park and down to Mt Stewart in the south, numbers grew from 4,316 to 8,518.

In other areas the numbers of horses actually dropped slightly — in the north-west of Kosciuszko, east of Tumbarumba, numbers fell from 1,616 to 1,113.

Horse numbers 'threatening pristine water catchments'

Stream bank damage in Kosciuszko National Park. ( ABC News: Alexandra Blucher )

There are currently no permanent horse populations within the ACT, which immediately borders the Kosciuszko National Park.

ACT authorities are determined to keep it that way, and cull horses found crossing the border into the Namadgi National Park.

The ACT's Environment Minister has called on both the NSW, Victorian and Federal Governments to take swift action on the "alarming" figures and "respect the science".

"Feral horses do not recognise state boundaries and the scientific evidence is clear — heavy-hoofed pests such as feral horses are damaging the landscape," Mr Gentleman said.

"The integrity of our high-country areas is vital to preserve the quality of Canberra's drinking water, and the water that flows from the Australian Alps, which contributes more than 30 per cent of inflows into the Murray-Darling system."

But he warned the NSW 'Brumby bill' was a roadblock that threatened to prevent serious change.

"They can't use lethal culling for horses, and that's a real issue as we see those numbers growing in Kosciuszko," Mr Gentleman said.

Despite the introduction and passage of the bill, its architect — Deputy Premier John Barilaro — previously suggested he was open to reducing the horses' numbers by 50 per cent.

Under the new figures, a cull like that across the total population would still leave more than 12,000 horses — higher than the population estimated to be living in the alps in 2016.

The NSW Government has previously rejected recommendations to reduce the number of horses in Kosciuszko to 600 over a 20-year period.