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"The effects of larger protection were absolutely evident in the recovery of the species," Dr Costin said.

A picture of "this most beautiful plant" adorns this home in Narooma to this day.

Sir Garfield was won over, switching his critical vote to back the end of grazing leases even though the park's administration was reliant on those rents.

Battlelines redrawn

But conservation battles are often ceasefires rather than permanent victories.

The Berejiklian government last week secured parliamentary approval to extend protection to wild horses in the national park based on their connections to the region dating back some 200 years when Europeans first arrived.

The bill overturned the 2016 draft management plan that recommended reducing the estimated 4000-8000 brumbies to just 600 over 20 years.

Dr Costin reckons numbers are more like 10,000, with a horse's impact 10-15 times that of a sheep.

Back in the late 1950s, "you'd be lucky to see a handful of brumbies in a day's ride", he said.

Catchment risks underestimated

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The plan to protect hard-hoofed animals in the delicate alpine areas prompted dismay from scientists at home and abroad.

Dr Costin said that, beyond the effects on rare endemic plants and animals,  the impact on water had largely been ignored. That's despite the powerful role played by the Snowy Hydro scheme in getting sheep and cattle out of the high country in NSW and Victoria.

As the 2016 draft report noted, "the need to protect these catchments was a significant factor in the
establishment of Kosciuszko State Park in 1944". It hosts the headwaters of the Snowy, Murray and Murrumbidgee rivers.

'Right balance'

A spokesman for NSW Environment Minister Gabrielle Upton said the government was "committed to striking the right balance between what’s good for both the heritage and the environment".

"The next step is the production of the Wild Horse Heritage Management Plan, which will consider factors such as catchment values," he said.

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But Labor's environment spokeswoman, Penny Sharpe said the Coalition had "failed to take into account any of the serious issues that are impacting on Kosciuszko National Park as a result of the unmanaged horse population. Water is another one of these."

Mehreen Faruqi, her Greens counterpart who also worked as an engineer in catchment management, said the government had "put our water resources at huge risk".

"With climate change making future water availability highly uncertain we must do everything to protect our precious water catchments," Dr Faruqi said.

Evidence that climate change is already having an impact includes less rainfall over south-eastern Australia since the 1970s amid rising temperatures, according to research by Graeme Worboys from the Australian National University.

Snowy Hydro has said little about the brumbies. Chief executive Paul Broad told Fairfax Media that "as a commercial entity, we do not take part in government's decisions or policy-making around environmental management" of the park.

Still, "while Snowy Hydro is not currently experiencing any direct impacts from wild horses, we can see the damage being done by them", he said.

'Adverse effects'

However, Dr Costin said the trampling with hard hooves was already creating deeper channels, allowing water to flow faster, speeding erosion.

The more turbid waters "can have adverse effects on the wear and tear on turbines" in the Snowy Hydro scheme, he said.

The peats and bogs, which now help spread the water and keep rivers flowing during winter, are also drying out. That also creates a worse fire risk during summer.

And, if the decades of tree replanting needed to repair sheep damage are any guide, future restoration to address the impact from horses will be expensive and lengthy.

"It's going to take a long time to recover," Dr Costin said.