A malfunction at a sewage treatment plant has sent raw waste flowing through properties and a reserve in the Coromandel resort of Whangamata.
The police say heavy rain triggered the overflow at the treatment station at Awarua Point, and the waste has been making its way into the harbour.
Council staff are trying to fix the problem, but the situation is not expected to improve before Sunday.
In the meantime, the police are advising people to avoid swimming in or near the harbour.
Auckland beaches contaminated
Beachgoers in Auckland are also being advised to avoid swimming after heavy rain caused sewage to overflow onto beaches on Friday night.
Auckland Council is recommending people stay out of the water for the next two or three days.
It says water tests to determine bacteria levels are being carried at Okahu Bay, Mission Bay, Kohimarama, St Heliers, Blockhouse Bay and Pt Chevalier.