Waikato Regional Council has rejected concerns a planned drop of 1080 poison on the Whangape wetland in north Waikato will contaminate water piped to Auckland.
The 1080 drop to control possums is being made in conjunction with landowners in the area and is likely to happen early next year.
Those against the drop believe it could seep into the Waikato River if it rains and the wetlands flood.
Campaigner Wyn Hibberd says the wetlands feed into the Whangape stream which in turn flows into the Waikato River close to the Auckland water pipeline intake.
The regional council says it has the approval of both the Department of Conservation and health authorities.
The council says it's confident water supplies won't be affected as the 1080 will be broken down into "harmless components" by the time it reaches the river.