Greens MPs are at war with environmental groups over a NSW government plan to extend protection to feral horses in the Kosciuszko National Park.

Deputy Premier John Barilaro surprised colleagues last weekend by ditching a draft wild horse management plan that would have cut the number of brumbies in NSW's biggest national park from an estimated 4000-8000 to 600 over 20 years.

Unusually for an invasive species, wild horses will be now be protected - in that park alone - for their heritage links dating back to colonial times, according to the bill now before Parliament.

In a first, the NSW Berejiklian government plans to legislate protection for an introduced species, the brumby, in a national park. Credit:Nick Moir

For the Greens, the policy reversal revealed tensions over animal rights versus the environment. The hard-hooved animals now range over about half the national park, damaging delicate upland flora and adding to the threats to endangered fauna, such as native frogs, lizards and crayfish.