No brumbies were removed from Kosciuszko National Park in 2018 despite it being the only method granted to parks staff to control feral horse numbers.
The year before, in the winter of 2017, staff removed 152 horses from Kosciuszko, with all but one later rehomed.
Non-lethal control of brumbies was a key part of the controversial law passed by the NSW government last year which granted the horse a "heritage status".
ACT Environment Minister Mick Gentleman said the lack of action was "irresponsible" and ACT Parks manager Brett McNamara said it was "alarming", with concerns a nearby 4500-strong population will soon enter and destroy the ACT's water catchment areas near Namadgi National Park.
The NSW Office of Environment and Heritage confirmed no horses had been removed from Kosciuszko in 2018.
"This is a wake-up call for us," Mr McNamara said.
Mr McNamara is concerned an unchecked feral horse population across the ACT/NSW border will eventually get into Canberra's Cotter water catchment areas, destroying the natural environment and robbing the capital of its main water source.
By trashing the catchment it's feared the horses will also destroy habitats for the critically endangered northern corroboree frog, as well as other native species like the alpine tree frog or broad-toothed rat.
Horses are not native to Australia and the ACT government has strict lethal controls in place for feral horses but is powerless to act on the issue outside of its borders.
"Last count there was 4500 horses within the northern section of Kosciuszko National Park," Mr McNamara said.
Feral horses in Kosciuszko were given protections by the NSW government last year after it passed its so-called "Brumby Bill", which would see a new horse management plan drafted allowing only non-lethal methods to control their numbers, including rehoming.
"A political decision has been taken with only a short-term view," Mr McNamara said.
In 2016 an independent advisory group told the NSW government to reduce the brumbies' Koscisuzko population from the then 6000 to 600 with control methods, including aerial culls, over 20 years.
"We call for the NSW government to go back to the old horse management plan that went through extensive consultation," Mr McNamara said.
"Repeal the Brumby Bill and go back to the [old] plan."
Mr McNamara said the damage from feral animals, including pigs and deer, would be incremental but the effects on Canberra could be seen inside a decade.
He pointed to the current fish kill crisis gripping the Murray-Darling Basin.
"We are all aghast as we see all these dead fish. That didn't happen overnight, that happens over generations and over political cycles."
He also warned the overabundance of feral horses would see them starve with prolonged inbreeding spelling further problems.
Environment Minister Mick Gentleman said the NSW government's "irresponsible policy is putting pests ahead of our native animals, eco-systems and drinking water".
"The NSW government must stop its recklessness and follow the advice of scientists and rangers, rather than rose-tinted nostalgia," Mr Gentleman said.
The wetlands that feed Canberra's drinking water catchments were declared endangered environments by Mr Gentelman earlier this year.
In the winter of 2017, Kosciuszko's parks staff removed 152 horses from Kosciuszko, with 151 horses rehomed and one euthanised by a rehoming organisation.
Mr McNamara said that wasn't enough.
"It's the tip of the iceberg. In 2014 it was estimated there were 9000 horses over all the Australian alps," he said.
Wilderness Society policy director Tim Beshara said the federal government had pressed pause on their responsibilities under Australian environment law.
"Where the hell is the Federal Environment Minister here?" Mr Beshara said.
"She has a responsibility to protect endangered species and a responsibility to protect the heritage values of the Australian Alps."
Federal environment minister Melissa Price originally ignored Mr Gentleman's request to meet and intervene on feral horses in November last year.
When her office was contacted by The Canberra Times in January this year, her spokesman said she was now preparing a response and would be happy to meet.