Brumbies protected from lethal culling in NSW will be killed if they cross the border into the ACT.

The NSW Parliament passed legislation on Wednesday that banned lethal culling of wild horses in Kosciuszko National Park, outraging scientists and environmental groups as the state government reversed a plan to cull an estimated 90 per cent of brumbies in the park over 20 years.

Brumbies that wander into the ACT will not enjoy the same protection from lethal culling that they have been afforded in Kosciuszko National Park. Credit:Karleen Minney

But brumbies that roam into the territory won't enjoy the same protection, with the ACT Parks and Conservation Service concerned about the impact they could have on Canberra's drinking water, as well as threatened plants and animals.

One of the capital's main water sources of drinking water, the Cotter catchment, is in the high country in Namadgi National Park.