No rats or mice have been detected in tracking tunnels in the Catlins recently, suggesting last year's 1080 aerial drops in the area have been a big success.
More than 10,000ha was targeted in the Catlins in late November.
Department of Conservation (Doc) ranger Cheryl Pullar, of Owaka, said before the poison drops. 24% of tracking tunnels recorded rats.
The tracking figures represented ''the best possible result'', she said.
''While we can't say we've eradicated rats ... tracking at 0% means that they are undetectable with our methods.''
The 1080 drop last year had ''an excellent effect on the rats, [and] a good kill on the mice, which substantially reduced the stoat population.''
But proof of the success of the 1080 drops - part of the nationwide ''Battle for our Birds'' campaign - would not be known until the birds became more territorial and reliable counts were possible.
In the meantime, rat numbers were expected to remain low.
''They won't have a chance to increase again until next spring, and depending on the food supply next spring that will show how fast or slow they will recover to worrying levels,'' Ms Pullar said.
Murihiku biodiversity ranger Finlay Cox said the percentage of mice tracked had dropped from 79% to undetectable levels.
Anecdotally, there was ''heaps more bird life'' in the area.
''And given the mohua population in the Catlins, continued control during mast and partial mast events is essential.''
Ms Pullar said a 1080 aerial drop in the Blue Mountains conducted by TBfree New Zealand also produced good results.
In the roughly 6000ha targeted area, rats had been tracking at 16% but that was now down to 5%.
