Few people know the Kosciuszko National Park better than Dave Darlington.

Now retired, he began working there in 1987, and held a number of jobs, including being the manager of the entire park in south-eastern NSW.

Mr Darlington said he saw the park become transformed by feral horses — known as "brumbies" — over the years.

"Twenty or 30 years ago, there were a lot of streams out there that if I was thirsty, I'd just put my face into the stream and drink the water. I can tell you now I don't do that — and few people do — because the water is just not pure like it used to be," he said.

Mr Darlington said horses and other feral animals like deer are compressing the loose moist soils of the park and squeezing them dry, which kill ecosystems that rely on constant moisture.

The resulting erosion, combined with defecation, has left many of the streams polluted, he said.

'Wilful disregard' for science

In April, these concerns led the NSW Government's Threatened Species Scientific Committee to table the recommendation that feral horses be listed as a "key threatening process" alongside things like climate change, invasive weeds and exotic fungi.

Ecologist David Watson quit his position as a scientific adviser to the NSW Government in protest of the new legislation. ( Twitter: David Watson )

Despite those worries, on Wednesday the NSW Government passed legislation that protects feral horses: the Kosciuszko Wild Horse Heritage Bill 2018.

The Bill seeks to establish a "sustainable wild horse population" inside the national park, and recognise their heritage value.

Ecologist David Watson, a member of the scientific committee for more than three years, quit the next day in protest, saying the Environment Minister Gabrielle Upton was showing a "wilful disregard" for science.

"I can no longer give my time to a system that is clearly broken," he told the ABC.

A spokesman for Ms Upton told the ABC: "Minister Upton wholeheartedly supports the Bill".

Mr Darlington said he understood people's love for horses and he shared it.

"But if you have an attachment to the park as it used to be — and an attachment to our native animals, then you can't have it both ways," he said.

State's plan 'impossible' to implement

Experts have said the Government's plan would be unworkable without lethal options because the feral horse population would boom.

Professor Don Driscoll, an ecologist at Deakin University, said as soon as control measures were lifted, the population would explode along its existing frontier.

"With no controls, the population will continue to increase until they reach the carrying capacity," Professor Driscoll said.

One of the brumbies relocated from the Burragorang Valley in the NSW Blue Mountains. ( Supplied: Russell Kilbey )

The horses currently live in about half the park, and in some of those areas they have reached their carrying capacity, Professor Driscoll said.

According to him, the brumby population would grow by about 20 per cent each year in some areas.

"It seems virtually impossible to implement," he said.

Mr Darlington agreed, noting that bigger populations require more effort to maintain.

"The number of horses you need to capture and remove is a greater challenge if the total number of horses is much much higher," he said.

He added that because of the park's ruggedness, catching the horses and getting them to trucks was impossible.

Additionally, if a fire or major drought affected the area, the animals would die in much larger numbers.

Horses could spill over into ACT and Victoria

Park managers in the ACT are seeing red over NSW decision.

"Personally, I was gutted when this announcement came through," said Brett McNamara, manager of ACT Parks conservation service.

He said he grew up in a rural setting and learned to ride a horse before riding a car.

"I have, and always will have, empathy for horses but it has to be in the right place at the right time," Mr McNamara said.

"Here in the ACT we have a zero-tolerance policy for horses in the Cotter Catchment."

ACT conservationist Brett McNamara is concerned about NSW feral horses making their way into other areas. ( ABC News: Mark Moore )

With Canberra getting 80 per cent of its water from that area, horse control has been given priority. When a feral horse is found, they are rounded up and euthanised.

"The concern we have from the NSW population is that they could double every three years," Mr McNamara said.

"We will have horses coming in from Kosciuszko," he said.

"It's the sheer numbers. At the moment, we only have a handful of horses.

"We could well have a situation where there are hundreds of horses and that takes us away from other projects like pig control and feral deer control as a result of this terrible piece of legislation."

It hurts horses in the long run, experts say

Without controls, populations would expand until they reached an area's "carrying capacity", according to Professor Driscoll.

He said the environment was unable to sustain any more horses, and the death rate would increase to the birth rate.

As a result, a lot of horses starved to death. And at that point, the horses would be completely transforming the local environment.

Mr Darlington said he loved horses, and did not want to see any killed ideally, however he saw no way of avoiding it.

"If you're really wanting to lower the population, the only way is to carry out ground shooting," he said.