A boil water notice is in place for the Horowhenua town of Tokomaru.
The Horowhenua District Council has made the order following a week of continuous rain that has flooded and muddied the Tokomaru river.
Until further notice, people connected to the town's water supply are asked to boil water for drinking, food preparation, brushing teeth and preparing baby formula.
The Horowhenua District Council said those with severely compromised immune systems, infants, pregnant women, and some elderly may be at increased risk.
Such people are asked to seek drinking water advice from a doctor.
Residents are advised to boil water for one minute to kill any bugs, and reduce water use where possible to help the local treatment plant cope.