Up to 15,000 cubic metres of raw sewage has flowed into an inlet just north of Nelson from a damaged sewer pipe.
The Nelson City Council says the leak, near Atawhai, was discovered on Sunday morning.
Its community relations manager Angela Ricker says no one should get into the water at the Nelson Haven.
"There are signs posted for people to avoid swimming until further notice and there is a permanent shellfish ban."
Ms Ricker says repair work is expected to be completed soon but it may take several tides to clear away the sewage.