The Hutt City Council has started urgent repairs to its main outfall pipeline, which is leaking.
It could take at least two weeks to fix, and treated wastewater is now being discharged into the Waiwhetu stream near the Hutt River mouth.
Hutt City Council's general manager of infrastructure, Bruce Sherlock, says it is not known yet how the leaks were caused. He says it will be a big job to fix, as there are five separate leaks on the main outfall sewer.
Mr Sherlock says the pipeline is a vital part of the Hutt Valley and Wainuiomata's wastewater system.