An accidental wastewater discharge is copping the blame for a popular Christchurch surf spot being off limits.

Signs have gone up at Sumner Beach, between Cave Rock and Scarborough, warning people not to enter the water because of pollution.

The Christchurch City Council says the discharge was caused by an air valve failing along the Esplanade yesterday afternoon, close to the Sumner wastewater pump station.

Reticulation and Maintenance manager Chris Mance says the warning signs will remain until testing confirms the water poses no health risk.

He says that could take up to 48 hours, so the sea will likely be a no-go zone til at least Friday afternoon.