Raw sewage is spilling into central Porirua waterways.
In a statement, Wellington Water said crews were responding to a wastewater pipe rupture in the gully below Tasman Close on Sunday evening.
"This rupture has caused untreated wastewater to enter the Kenepuru Stream in Bothamley Park, which flows into Porirua Stream at Mepham Place," the statement said.
"We are working to resolve the issue as quickly as possible. We advise people to avoid entering the Kenepuru Stream and Porirua Stream until water testing comes back clear and a full repair has been completed."
Bothamley Park was already closed to the public due to an ongoing wastewater renewal project and warning signs were being posted at potentially affected areas, Wellington Water said.
Regular sampling would be taken from Kenepuru Stream at Mepham Place and at the outlet of Porirua Stream, the statement said.
"The location of the rupture makes access difficult, and health and safety concerns make a repair complex," it said.
A geotechnical engineer had been contacted to complete an assessment on Monday morning.