Decisions made in Macquarie Street, Sydney, last year turned the tables on more than 100 years of careful ACT water supply protection and in Canberra, the minister responsible, Mick Gentleman, is fuming about it.

"I've called the Barilaro legislation reckless and most scientific advice fully supports that. It's incredibly dangerous to ignore what is happening now, and it will get a whole lot worse into the future if nothing is done," Mr Gentleman said.

In May last year the NSW government, with the key support of the Minister for Regional NSW and Member for Monaro, John Barilaro, passed a bill which stopped culling of wild horses, recognising the heritage value of the Snowy Mountains brumbies.

It protects them by identifying areas within the Kosciuszko National Park where populations can roam "without causing significant environmental damage", Mr Barilaro said.

The legislation also prohibits culling, endorsed "non-lethal" management, and says those which roam into sensitive alpine areas would be "relocated".

In another report from a recent public meeting in Jindabyne, he supported the removal of 50 per cent of feral horses from the Kosciuszko. It's a claim he later denied.

So as votes are cast in NSW on March 23, the concern in the ACT will be what action the incoming government may take. If Labor wins, it has pledged to repeal the so-called "Brumbies Bill". If the current government is returned, nothing appears likely to change.

The "do nothing" option has significant long-term ramifications for the ACT's water.

Just over the border in NSW, down in the plains to the west of the Brindabellas, wild horses are grazing around the water courses and churning the banks of the lowlands creeks into a muddied, turbid black mess.

There are foals at foot among them, and clearly pregnant mares. And there are some strange abnormalities to their heads and backs, almost certainly a result of inbreeding.

These big, heavy animals cover dozens of kilometres in a day and always keep moving to where water is readily available and the green pick is the sweetest.

They don't know or care the border between the two jurisdictions sits atop the Scabby Range just above them. Highly selective feeders by nature, the horses keep climbing the foothills, looking for green pick.

One, a stallion by its large and concentrated piles of dung, is a regular visitor to the ACT's highest peak, Mt Bimberi. Others have visited previously, and were trapped and removed under a strict ACT feral pest control management plan.

Up until last year, parks rangers on both sides of the border co-managed the wild brumby population.

Now the culling program which kept the NSW brumby population in check for decades has effectively ceased, with no horses shot or removed from the Kosciuszko National Park in 2018. They breed and in-breed, their hooves ripping into soft, peaty loam.

Reports have now emerged of the disruption feral horses are causing at camping grounds such as the popular Blue Waterholes, just over the border in NSW.

There are reports of children being kicked and injured, aggressive horses galloping through the campground and getting tangled up in guy ropes, and cars being damaged. People fearing being tramped in the night have abandoned their tents and swags and slept in their cars.

Atop the borderline, on one of the ridges pegged out by Harry Mouat, Fred Johnston and his survey team more than 100 years ago to protect Canberra's future water catchment, ACT Parks manager Brett McNamara is a worried man.

The ACT enjoys an enviable national reputation for its high water quality. The source of that water is high in the Brindabellas, among the alpine sphagnum bogs and fens.

These delicate, sponge-like habitats suck up the autumn and winter rain and slowly, ever slowly, release it down the mountain during spring and summer for it to trickle through the catchment and into our dams.

Such is the thick matting of the hummocks that the effects of erosion here are minimal and its underlying cool puddles of water, still present despite the unusual autumn heat, provide perfect protection for the native Corroboree Frog.

Mr McNamara's role is to protect this fragile habitat and believes the feral horse threat is a very serious one. An aerial survey is about to begin to find out just just how much the mobs have grown since the previous 2014 survey, which reported more than 4000 in the North Kosciuszko area alone, and nearly 9500 across the entire Australian Alps.

"Our water is so precious; this is one of those creeping issues which if we ignore it now, this delicate environment will be crushed underfoot and we will look back in five or ten years and ask ourselves: why the hell didn't we shout this from the rooftops at the time?" he said.