The NSW government has been accused of making Australia the laughing stock of the world after parliament passed a controversial bill to protect wild horses.

The legislation, given the rubber-stamp on Wednesday night, sparked anger among environmentalists and conservation groups, and led to the resignation of a member of a government-appointed panel of experts.

Professor David Watson announced his resignation from NSW's Threatened Species Scientific Committee on Thursday citing the "wilful disregard" the government has for science by passing the bill.

"Feral horse populations have a litany of negative effects on native plants, animals, and ecological communities," Prof Watson said in his resignation letter.

"Put simply, feral horses are incompatible with protected area management, a point made repeatedly in the days leading up to yesterday's decision."

NSW Deputy Premier John Barilaro argued the laws aimed to find a balance between managing sensitive areas of the Kosciuszko National Park and managing the feral horse population through humane population control measures.

"This will finally end years of speculation around the lethal culling of one of Australia's national icons," Mr Barilaro said in a statement.

If brumbies are found in highly-sensitive alpine areas of the park, resources will be allocated to relocate them, and if population numbers grow too high, they will be re-homed.

But conservation groups were scathing, with the Invasive Species Council saying it has turned Australia "into a global laughing stock".

"It will lock in pain and suffering for the horses, which, as their numbers grow, will eat out the bush and then starve, and gives the green light to environmentally destructive management of our national parks," chief executive Andrew Cox said in a statement on Thursday.

Ecologists, international conservation groups and the state's leading animal welfare body, the RSPCA, were among those who voiced opposition to the move, as well as NSW Labor and the Greens.

National Parks Association chief executive Alix Goodwin said it was an "international embarrassment" and vowed to fight to get the law repealed.

Meanwhile, the Victorian government is planning to rehome or cull about 2500 feral horses in the alpine region.

Australian Associated Press