The original NSW plan estimated the Kosciuszko National Park had about 6000 brumbies, and aimed to reduce those numbers by 90 per cent to just 600 over 20 years – before the policy reversal.

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Victoria's eastern alpine region has about 2500 feral horses. The proposed protection of these animals in the adjacent areas across the NSW border potentially undermines Victorian efforts to reduce their numbers.

Victoria has set a goal to remove as many as 1200 horses during the initial three-year plan to 2021.

"Based on current trapping rates for the Victorian alps, this will mean a significant trapping increase, up
to 10 times the average for the period 2007-17," the plan states. At least 200 horses must be removed each year to prevent numbers from increasing.

“Feral horses cannot be allowed to run rampant in [Victoria's] Alpine National Park,” Lily D’Ambrosio, Minister for Energy, Environment and Climate Change, said.

“We have developed a strong plan to protect this treasured part of the world, and we call on the NSW and the Turnbull governments to support it.”

Trapping will be the principal method to control the population. Shooting will not be used during the three-year plan,  although consultation will be undertaken in the third year about "additional techniques" if other efforts are needed, the report said.

Fairfax Media sought comment from John Barilaro, the NSW Nationals leader and Deputy Premier, and Gabrielle Upton, the state's Environment Minister.

Josh Frydenberg – who initially dubbed brumbies "a bit of pest" that should be controlled, before backing the NSW's protection plan for the animals – was also approached for comment.

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Andrew Cox, chief executive of the Invasive Species Council, said Victoria had undertaken an "exhaustive public consultation" to settle on its plan.

"When the public understands the difficulties faced by Parks Victoria, they would realise these measures are needed," Mr Cox said, adding NSW had "gone the extreme path".

Labor NSW, which opposes the government's protection plan for the horse, said NSW had "again demonstrated how it is willing to ignore science in the pursuit of the desires of its political donors”.

“Our fragile alpine regions don’t recognise state boundaries," Ms Sharpe said. "A joint and consistent approach is the only way to protect the environment, tourism and jobs in this region.”