An aerial 1080 drop is planned over 19,000ha at the head of Lake Wakatipu to control soaring rat numbers, stemming from the beech forest's ''mega mast''.

Department of Conservation (DoC) manager Geoff Owen said up to 15,000 seeds were falling per square metre, giving rise to predators which could potentially extinguish a local population of critically-endangered mohua, or yellowhead.

More than 1000 traps had been laid in the Caples, Dart and Routeburn valleys but due to the massive seed fall this year, more intensive protection was required, Owen said.

The ''mega mast'' had provided a bounty of food for native insects and birds, but also rodents, whose populations could expand rapidly, he said.

Advertisement

''When the seed runs out, they turn to our vulnerable species''.

Next month's 1080 operation was part of DoC's national Tiakina Ngā Manu (formerly Battle for Our Birds) programme.

Department of Conservation manager Geoff Owen. Photo / File
Department of Conservation manager Geoff Owen. Photo / File

A second drop may be required this summer if rodent numbers rebounded quickly due to the massive amount of food available.

While the focus for this year's Wakatipu operation was to protect mohua, other species such as pekapeka (southern long-tailed bat), whio (blue duck), New Zealand robin/toutouwai and kākā would also benefit, Owen said.

The 1080 drop would begin during the first period of good weather after August 1, and run over two separate days.

Non-toxic pre-feed pellets would be laid on day one to get pests used to the pellets as food.

About a week later, weather permitting, toxic bait pellets would be laid.

Owen said all walking tracks in the control area would be closed during the pre-feed and toxic bait operations for up to one day each time.

Advertisement

Deer repellent would be used with the 1080 cereal bait to minimise the risk to deer across all three valleys, he said.

''This is a special area close to many locals' hearts.

''Anyone who has visited these valleys recently will have seen and heard the beautiful birdsong, brought about by sustained predator control.

''But it only takes one heavy mast year to lose these gains.

''If we do nothing we are facing local extinction - we need to do all we can to prevent this.''