The Department of Conservation (DoC) hopes controlling tahr numbers in the Southern Alps will slow the rate of devastation in precious alpine areas.
The wild Himalayan herd has been reduced by 12,000 with still more work to do.
The Zora Canyon is a rugged and imposing wilderness area in south Westland. It's also a favourite grazing place for the Himalayan tahr.
For the last 20 years, DoC has been monitoring the impact tahr are having on this remote and awe-inspiring canyon.
A photo taken in 1999 shows large snow tussocks - today in the exact same spot they're all but gone.
"The tall tussocks are the trees - it's no longer a forest," DoC ecologist Ingrid Gruner says.
The impact tahr are having can be seen from the Rakaia River in the north to Haast in the south.
Moving in mobs of up to 40 at a time, they're quick and agile in this rugged landscape.
On the Ben Oahu Range, ecologist Brian Rance can see tahr signs - revealing they've been through it recently.
"On the ridges where animals have passed through you can see these shaving brushes," he says.
DoC has made a concerted effort to reduce the tahr population in the Zora Canyon with up to 300 culled just last year.
It's part of control work that last year saw the 35,000-strong tahr population on Conservation land reduced by 12,000.
DoC wants to get the tahr numbers down to 10,000 on Conservation land.
Gruner is hoping those efforts will give places like the Zora Canyon a chance to recover.
"It is vitally important, I find it heartbreaking to see this, because of an introduced species," she says.
An introduced species with a strong foothold in the Southern Alps is leaving its mark, one that could take a lifetime to recover from.