Sewage has been discovered spilling into Lake Wairarapa after a valve used to drain the Featherston oxidation ponds was tampered with.
Greater Wellington Regional Council says the leak was discovered during routine maintenance and could have happened any time between 5 February and 11 February.
The council has reported the leak to police, the medical officer of health and landowners downstream of the ponds.
Warning signs have been put up in the area, and the council has taken samples.
The council says it is a serious incident, which could have adverse effects on water quality and native fish in Lake Wairarapa.