Residents in Taumaraunui and nearby Manunui are being told to boil their water after faulty filters were discovered at the Matapuna water treatment plant.
Ruapehu District Council said the filters were slowly blocking and affecting the amount of water the plant could produce.
The impending arrival of Cyclone Lusi this weekend prompted the decision to remove the filters - the last step in the treatment process - so reservoirs can fill before the bad weather hits.
However the council said the water was still being disinfected and the warning was precautionary.
The boil water notice is likely to remain in effect until at least Monday.