A two-week operation involving a 1080 drop on Auckland's Hunua Ranges - where 60 percent of the city's water supply comes from - begins today.
The drop is part of a strategy will target a plague of rats and possums threatening native species in the ranges.
It will be made in three stages over two weeks, starting today with non-toxic bait to get the pests used to it, and the poison will follow in about a week.
Auckland Council said a total of about 50 tonnes of bait would be dropped over 21,500 hectares.
The Council's biodiversity manager Rachel Kelleher said there would be a lot of testing after the drops.
"Monitoring the actual pests that we are targeting, through to monitoring of non-target species, or species we are trying to protect, like the kōkako or Hochstetter's frogs.
"We also have a very sensible water monitoring programme and that programme is both of public drinking water reservoirs, a number of streams we are looking at testing and there is also a small number of sites we are testing around private drinking water takes."
She said the main water tests would be done within eight to 72 hours of the poison being dropped.
Chair of the Wharekawa marae, at foothills of range, Tipa Compain said surrounding iwi supported the drop.
He said they had been involved in consultation for nine months, and were confident the risk of 1080 was minimal.
"We've had in-depth discussion with the council around the technical side of the 1080," Mr Compain said.
"Just the make up and the composition in terms of how much 1080 is being dropped, and if it does get into the waterways, it's been explained to us how they are going to manage keeping that 1080 out of the supply of the dam."
Mr Compain, of Ngāti Whanaunga descent ,said the Auckland Council had reassured tāngata whenua that the 1080 pest control campaign will be closely monitored and that local iwi were satisfied with that.
The council expects to know within six to eight weeks whether the drop has been a success.