Brumbies in Kosciuszko National Park are trampling the habitat of a critically endangered fish and could soon wipe out the entire species, a University of Canberra researcher has warned.

The NSW government's controversial Kosciuszko Wild Horse Heritage Bill 2018 was passed late on Wednesday, despite widespread criticism from scientists around Australia and the world.

The legislation bans the lethal culling of brumbies in the national park because of their cultural significance, reversing the NSW government's plan to cull an estimated 90 per cent of wild horses in the park over 20 years, leaving just 600.

University of Canberra associate professor Dr Mark Lintermans has blasted the decision, saying it "flew in the face of science" and warning that it could spell the end for the stocky galaxias freshwater fish.