The final stage of the 1080 bait drop on Auckland's Hunua Ranges is underway.
It is part of a strategy to tackle a plague of rats and possums threatening native species in the area.
Officials said it was the first drop of this scale in the ranges for at least 20 years.
Today's operation by helicopter is covering the final 11,000 hectares of the Auckland Council's 21,000 hectare forest.
A total of about 50 tonnes of the controversial 1080 bait is being used.
The ranges supply 60 percent of the city's water supply, but the council said it was doing extensive testing on the safety of the water following each stage of the operation.
The Hunua Ranges, Waharau and Whakatiwai regional parks are closed until at least the end of the week so that the tracks can be cleared of bait.