An environmental group claims the wild horse management plan in Kosciuszko National Park will fail unless there is a significant increase in the number of brumbies being removed.
It follows the release of the first information about the implementation of the plan to reduce the horse population from 14,000 to 3,000, which was adopted in November last year.
The National Parks and Wildlife Service removed 334 horses between February and June this year but would not say what happened to the animals.
The Invasive Species Council's James Trezise said that number was not enough to keep up with the breeding rate of the horses, let alone meet the plan's objectives.
"We have 14,000 feral horses in Kosciuszko National Park and that population grows by 20 per cent every year. Removing a fraction of that is not going to stop that population actually growing," Mr Trezise said.
"The population of feral horses in Kosciuszko will just simply balloon, so there is a clear need to step up and ramp up efforts to get a much larger number of horses out of that park by a variety of control measures."
The National Parks and Wildlife Service said the outcome reflected a "phasing in" of control operations.
Mr Trezise said the recent State of the Environment Report highlighted the impact of invasive species on native ecosystems.
"If we don't tackle feral horse in Kosciuszko it will mean the irreversible loss of critically endangered native ecosystems and species," he said.
Call for more transparency
Brumby advocates have criticised the service for not releasing more information about what happened to the horses once they had been removed from the park.
Former Monaro state MP Peter Cochran said the service was doing "whatever they can to conceal" management plan operations.
"They're not being accountable, they're not publicly releasing all of the figures, simply because they are ashamed of the way the management plan is being operated," Mr Cochran said.
"The general public shouldn't have to go to Question Time in the parliament in order to get information out of the National Parks and Wildlife Service.
"They need to be accountable as to what they're doing with the horses, what the result of the management plan is, and that is where the horses are going."
The plan also includes environmental rehabilitation in areas of the park where horses have been removed.
In a statement, the service said it would not reveal more details, in order to protect the safety of staff, contractors, visitors and the wild horses following trap tampering and threats.
"During the reporting period, some matters involving interference with control operations, and threats against NPWS staff, were referred to the NSW Police," the statement said.
