Peter Cochran, president of the Snowy Mountains Bush Users Groups and a former NSW Nationals MP, said the government is set to reject key measures outlined in its draft management plan released a year ago.

The draft called for the elimination of 90 per cent of the estimated 6000 wild horses in the national park over 20 years to limit impacts on the sensitive upland environment. Ground-based shooting was one proposed control method.

"They have ruled out all shooting - aerial and ground - unless there are exceptional circumstances," such as an injured animal, Mr Cochran told Fairfax of the revised plans. He attended last month's Nationals conference in Broken Hill.

The rate of brumby culling would reportedly be less and the eventual population size greater compared with the 2016 draft.However, Environment Minister Gabrielle Upton said the government was yet to finalise the management plan: "I am sure we can get the balance right and the new plan will also have wide community support."

Securing wide approval, though, won't be easy, particularly from scientists. In a letter last week to the Premier Gladys Berejiklian, the Ecological Society of Australia's president Don Driscoll said horse numbers may already be underestimated and "the problem gets worse the longer a decision to rapidly reduce horse numbers is delayed".

Professor Driscoll urged the premier to "consider use of aerial culling, and at the very least to implement substantial ground-shooting" to cut the timetable of herd reductions to "within a few years", rather than a couple of decades.

At risk are critically endangered species such as the corroboree frog, alpine water skink, and the broad-toothed rat, according to the letter.

Graeme Worboys, an honorary associate professor at the Fenner School at Australian National University, said wild horses had already degraded about a third of the wetlands in the Australian Alps, with a bigger impact likely in the Kosciuszko region.

"The degradation means less water when you need it most," Dr Worboys said, adding Prime Minister Malcolm Turnbull's "Snowy 2.0" plan to increase storage in the hydro scheme could also be at risk.

"It is just dumb to allow the catchments to be trashed when the water is so important and when the management solution is so simple," he said.

Mr Cochran, who runs horse rises into remote regions, said feral deer and pigs may be doing more damage than horses to the streams.

The brumby should be valued for the historical links to early settlers and popular legends such as The Man From Snowy River, and military roles such as with the Light Horse Brigade. "It's ingrained in our culture now," Mr Cochran said.

Labor's environment spokeswoman Penny Sharpe said the opposition recognised "the iconic nature of the horses" but also "listens to scientists and technical experts".

"Labor supports proper resourcing of the National Parks and Wildlife Service, the humane treatment of the horse population and we have a open mind on the methods required to reduce the current population," she said.

Mehreen Faruqi, the Greens environment spokeswoman, said the party had "deep concerns" about the draft plan, especially the cruelty involved in trapping and shooting the horses.

"The proposed 90 per cent reduction in brumby numbers was extreme and seemed to be just a number plucked out of the air," Dr Faruqi said, adding non-lethal methods, such as fertility control, should be given priority.