Residents in the Wairarapa town of Featherston have been told they need to keep boiling water for at least a week after floodwater contaminated their supply.
The weekend's unusually high rainfall from Cyclone Dovi caused surface water to enter the Boar Bush Reservoir.
In an update this afternoon, Wellington Water confirmed crews have isolated the reservoir from the rest of the Featherston drinking water network.
"The network will be flushed, in the next couple of days, to ensure it is clear of any contaminants. When this is happening, users may experience lower pressure than normal", the company said in a statement.