Tens of thousands of litres of untreated wastewater has been discharged at Christchurch's Sumner Beach.
Christchurch City Council is warning residents to stay out of the water due to the accidental discharge.
They say the discharge was caused by an air valve failure near the Sumner wastewater pump station on Tuesday.
"We estimate between 22 cubic metres and 36 cubic metres," a council spokesperson said.
"Some of this would have been sucked up by a 'sucker truck'. The majority would have been swept out to sea as it was high tide at the time."
When converted, 36 cubic metres is equivalent to 36,000 litres.
Pollution signs will remain on the beach until testing proves the site is safe.