"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.