An increase in rat numbers has prompted the Department of Conservation (DOC) to push ahead with planned 1080 operations in the South Island.
Conservation Minister Nick Smith announced in January that DOC would aerially drop 1080 poison across 700,000 hectares of conservation land, mainly in the South Island, to curb rat and stoat numbers.
In Kahurangi National Park - the largest of DOC's planned 1080 sites at 270,000 hectares - monitoring found rat numbers climbed between May and August.
Landcare Research wildlife ecologist Dr John Innes said although mice and rats were able to breed several times a year, they generally did not breed in winter because food was sparse.
In years that beech seed was abundant, that extra food allowed the rodents to continue breeding throughout winter and greatly increased their numbers.
Although stoats only breed once a year, the increase in rodent numbers allowed stoats to produce more offspring the following summer, Innes said.
If rat numbers were allowed to increase unchecked, a stoat plague next year was likely with possible disastrous effects on native wildlife, he said.
Aerial 1080 drops were expected to substantially reduce rat numbers, with stoat numbers following suit as the carnivores ate poisoned rat carcasses.
DOC scientist Graeme Elliott said there were about 27 potential sites, but some of the planned operations would not go ahead because rat and mice numbers had not increased to concerning levels.
Six operations were completed last month, including Iris Burn Valley and Waitutu Forest in Fiordland National Park and Waikaia Forest in Southland.
Other areas like Kahurangi would be treated between now and November, with exact timing reliant on weather.
Elliott said the number of native birds killed by 1080 was "tiny and often zero".
The risk to birds was outweighed by the effect of unconstrained predator numbers, he said.
Kea especially were vulnerable to predators because they nested on the ground, in cavities or among tree roots.
Elliott said after a high beech seed year, kea had "appalling" rates of nest success and without intervention populations would continue to decline following successive rat and stoat plagues.
Confirmed 1080 drop sites for Sept-Nov Kahurangi: Cobb, Gouland, Anatoki, Wangapeka, Oparara Marlborough Sounds: Mt Stanley Lewis Pass: Te Maruia-North/South Nelson Lakes: Rotoiti Waimakariri: Hawdon-Poulter-Hurunui South Westland: Abbey Rocks, Landsborough-upper Otago: Mararora-Young, Matukituki-West/East, Wilkin Murihiku: Catlins, Blue Mountains Fiordland: Hollyford-Lower Whanganui: Waitotara