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. 

"When the flushing process has been completed, we take water samples for testing to confirm the water is safe to drink. We will monitor the network for at least three days to ensure the system is clear. 

Wellington Water manages water assets for the Hutt, Porirua, Upper Hutt and Wellington City councils, South Wairarapa District Council and Greater Wellington Regional Council. 

It's anticipated a boil water notice for Featherston residents will remain in place until early next week. 

"Our crews are working as quickly as possible, and we will provide regular updates on our progress. We know this situation is frustrating for you and we appreciate your patience while we fix this issue." 

The boil water notice applies to all businesses and residents connected to the urban water supply. 

Water must be boiled for drinking, making baby formula, juice, or ice, washing fruit and vegetables, preparing food or cooking, or brushing teeth.