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A groundbreaking trial to double the dose of aerial 1080 poison in a small area south of Haast has eliminated "all" 2000-odd possums, according to promoters.
They said the trial also got rid of all, or nearly all, rats in an area wedged between the Arawhata and Jackson rivers.
Plans are now being made to move from the trial stage and try to permanently eradicate pests from another area in South Westland.
Zero Invasive Predators (Zip) did the double pre-feed of non-toxic baits at Arawhata, followed by a double dose (4kg a hectare rather than 2kg) of poison
Zip chief executive Al Bramley said they had intensively searched a 400ha core of the trial area and after 50 days they did not find a single rat or possum.
They then extended it to 65 days, and still did not find one.
Mr Bramley said they had five people working inside the area full-time "to convince ourselves if something is there, we will find it".
Although they were ready to go in with a second poison drop, in the end they did not need to.
Zip believed the area had been home to about 2000 possums, and were "really confident" none were left. They have slightly less certainty with the rats.
Zip is now looking for a new site in South Westland to tackle next year, this time backed by the Southern Alps to create a third natural boundary.
This time the aim is not just eradication, but to hold it that way. The area will be far larger, probably around 10,000ha.
"We want to create a possum-free landscape in South Westland."
The Arawhata test area is open to reinvasion and will not be maintained.
Possums only breed once a year and will not reinvade quickly.
Rats move quickly and are expected to be back to their full population within 12 months.
Mr Bramley said their trial had big implications for predator control work.
"It could change the game. Do it once, do it right. And you shouldn't have to do it again," he said.
By Laura Mills