Wellington Regional Council has stopped taking water from a catchment area north of Upper Hutt because of a 1080 poison drop.
Helicopters dropped the baits on Friday in parts of the Kaitoke Regional Park and the Akatarawa saddle area to control possums.
Half the water supply for the Wellington urban area comes from the Hutt Water Collection Area, drawn at the Kaitoke weir.
Water for Upper Hutt, Lower Hutt, Wellington and Porirua cities is now being drawn from the water storage lakes at Kaitoke.
Taking water from the Hutt water collection area will resume when the Medical Officer of Health gives the all clear.