Auckland Council is reassuring residents drinking water won't be affected by a dumping of 1080 poison.
Hunua Ranges, Waharau and Whakatiwai regional parks and tracks in Auckland are closed while the poison is laid to kill pests.
Aerial application of poisoned bait over a section of 12,500 hectares began on yesterday morning.
Areas would be closed for up to five days, while the tracks are cleared, the council says.
Water supply reservoirs in the area have been isolated from the public water supply.
They won't be returned to service until consecutive samples show no 1080 has been detected, and the Medical Officer of Health signs it off.
Young children and dogs were particularly at risk of 1080 poison and people in the area should heed warning signs, the council says.
Dogs owners in the vicinity should watch them carefully and prevent them scavenging carcases.