People are being warned to stay out of the water near Sumner beach in Christchurch, due to a wastewater overflow.
Warning signs have been put up on the stretch of beach where the water was polluted.
A council spokesperson said the cause of the overflow was being investigated.
Samples were being taken to check bacteria levels in the water.
The spokesperson said the warning signs would be removed from the beach once the tests confirmed the water posed no health risk.
Sumner beach remains open to the public, and the stay out of the water message applies only to the stretch of beach where signs have been put up.