An aerial 1080 operation to control rat numbers across 11,000 hectares of the Dart and Routeburn valleys is scheduled to start in a fortnight.
The Department of Conservation (Doc) operation will start during the first period of good weather after September 6, and take place on two separate days.
Wakatipu operations manager Geoff Owen said monitoring had shown rat numbers had risen more than expected this winter.
Tracking tunnel data in mohua (yellowhead) habitat showed rat numbers persisting at 50% since May.
When the numbers exceeded 30%, the area's mohua population was at risk of extinction without further pest control.
``The current rat numbers will overwhelm our network of over 1000 traps in the valleys.''
Pekapeka (southern longtailed bat), whio (blue duck) and kaka populations would also benefit from the operation.
Non-toxic pellets would be dropped on the first day to accustom pests to the pellets as food, followed by toxic pellets about a week later, depending on the weather.
All walking tracks inside the drop area would be closed on both days.
The pellets will contain deer repellent.
