The Berejiklian government must reconsider its bill to protect wild horses in the Kosciuszko National Park because of their "significant negative environmental impact", according to scientists who compiled a 2016 management plan for the animals.

In a letter to Premier Gladys Berejiklian and Deputy Premier John Barilaro, five of the eight-member Independent Technical Reference Group that had called for a cut in feral horse numbers said "rigorous scientific and technical advice" should apply to their future management.

Members of the NSW advisory panel on wild horses say the government has taken the wrong course. Credit:Karleen Minney

"We are extremely concerned that the proposed bill, in its current form, will make it impossible

to conserve the unique environmental values of Kosciuszko National Park," the letter said.

The group noted the bill before the NSW Parliament aimed at protecting the heritage value of the brumbies would potentially damage those values in the state's biggest park by over-riding the National Parks and Wildlife Act 1974.