At present, wild horses that enter the ACT are lured into trap yards with salt licks, then sedated and euthanased with firearms.

However, Environment Minister Mick Gentleman fears NSW laws protecting brumbies from lethal culling in Kosciuszko National Park could result in increasing numbers of the animals moving into Namadgi.

"If horse populations in Namadgi National Park were to significantly increase by animals dispersing from a growing population in Kosciuszko National Park, our current methods would quickly become impracticable," a spokesman for Mr Gentleman said.

"It is likely that aerial shooting ... would need to be introduced."

Those comments come as Labor backbencher Tara Cheyne prepares to move a motion in the ACT Legislative Assembly on Wednesday, calling on fellow MLAs to condemn the "reckless and irresponsible" legislation passed by the NSW government in June.

According to Mr Gentleman's spokesman, a significant number of the estimated 6000 feral horses in Kosciuszko National Park are in the north-eastern part of the park, which borders the ACT.

He said 24 feral horses were removed from Namadgi National Park between 2007 and 2011. None are believed to be in the park at present, with only the odd transient brumby wandering in.

Ms Cheyne said she feared Kosciuszko's wild horses would pollute the Cotter catchment, Canberra's main source of drinking water, if they came across the border.

"We know that horses don't know where our borders are," she said.

"We know that the horses get into water and that they're pretty good at destroying creek boundaries.

"The impacts for us could be severe."

Ms Cheyne said it was particularly irresponsible of the NSW government to pass legislation protecting brumbies in Kosciuszko when there were flow-on effects for the ACT and Victoria.

Her motion will also call on MLAs to make it clear to the NSW and federal governments at every opportunity that the NSW legislation should be reversed.

In an August 20 letter to Mr Gentleman, seen by The Canberra Times, NSW Environment Minister Gabrielle Upton acknowledged there was evidence suggesting brumbies were impacting on the environmental values of Kosciuszko National Park.

Ms Upton insisted in the letter that finding a balance between protecting the environment and the heritage value of the brumbies was challenging, but possible.

"The new NSW legislation provides the framework for delivering this balanced outcome," Ms Upton wrote.

"The Act requires a wild horse heritage management plan be prepared that will identify where and how sustainable wild horse populations can be maintained.

"Where populations need to be reduced, capture and rehoming will be a priority and fertility control methods investigated."