The Omaui Landcare Charitable Trust recently carried out a 1080 drop in a 200ha area of bush targeting possums, stoats, rats and feral cats.
Trustee John Collins said the 1080 drop was the first the group had done with pest control contractor Tony Leith.
"It was very successful," Collins said.
Environment Southland conducted pest tracking work on the reserves using tracking tunnels and residual trap catch, which gave an indication of the population, Collins said.
Figures before the poison operation showed the possum population was at 20.83 per cent, which dropped to 1.3 per cent after the 1080 drop, Collins said.
Rats had dropped down from 44 per cent to 4 per cent, he said. The percentages relate to catch rates in traps.
There was no way of assessing the population of feral cats or stoats but the group assumed the results would be similar to the rats and possums.
Stoats and feral cats were likely to succumb to secondary poisoning when they ate animals that had died from poisoning, he said.
The trust is made up of a core group of Omaui residents whose aim is to improve the health and integrity of the forest by reducing the numbers of pests.
The group had cut around 10km of trap lines in the 200ha area, Collins said.
"We had teams of people out here for over a year."
"We're working closely with the Department of Conservation and Environment Southland. We've got a complete plan to eradicate pests and slow down the re-invasion into the reserve."
Since 2014, the Trust has received $76,857 in funding from the DOC's Community Conservation Partnerships Fund to carry out pest control on the DOC Scenic Reserve and Invercargill City Council Reserve.
Trustee Irene Schroder said the input from Environment Southland had been huge.
"All the meetings were on Sunday nights and they were always here and there was always more than one person. I can't speak highly enough of them."
In February they would need more volunteers as they went on to a weekly rotation checking traps, Collins said
During February and March, baby possums were kicked out by their mothers and made to find their own space, he said.
"That's when we're expecting a re-invasion."