A wastewater spill into Diamond Harbour earlier on Saturday, was caused by a burst drinking water main that damaged a nearby wastewater pipe, Christchurch City Council says.

The incident led to untreated sewage entering the harbour and prompted health warnings for Diamond Harbour, Purau Bay, Church Bay, and Charteris Bay.

Christchurch City Council Head of Three Waters Gavin Hutchison said a drinking water main burst first, scouring away surrounding material and triggering the wastewater failure.

“The burst caused approximately 42 cubic metres of material to scour away, the majority of which was clay. This impacted a neighbouring 100 metre gravity main wastewater pipe, which then also broke.”

Hutchison said the drinking water main was kept under pressure while repairs were made to the wastewater pipe to avoid contamination of the water supply.

“Approximately 30 cubic metres of wastewater is estimated to have spilled during this time,” he said.

“A significant portion of the total volume of water discharged to Diamond Harbour was drinking water,” Hutchison said.

“Approximately 120 to 240 cubic metres of drinking water is estimated to have discharged during this time.”

Hutchison said the council had reviewed the incident and confirmed the cause was a blown water main.

“Blown water mains are not uncommon, however a blown water main taking out a wastewater main does not happen very often,” he said.

“With large networks, such as Christchurch and Banks Peninsula, there will always be pipes that need to be renewed and we have a significant renewal programme in place.”

“In general terms, the network performs well and when there are failures the council’s contractor responds quickly to complete repairs,” he said.

Council sampling carried out on Saturday and Sunday showed either no presence or very low levels of Enterococci, an indicator bacteria used to test for wastewater contamination.

“This was at the overflow point and either side of the overflow point along the coast,” Hutchison said.

He said wider sampling was being carried out by Environment Canterbury.

On Saturday, Health New Zealand issued an alert, warning the water was not suitable for recreational use.

Public Health Medicine Specialist Dr Imogen Evans said people should avoid swimming in the affected areas.

“Water quality at the affected site is not considered suitable for recreational uses including swimming because of the risk to health from the bacteria and other pathogens,” Evans said.

“Water contaminated by human matter may contain a range of disease causing micro organisms such as viruses, bacteria and protozoa.”

Health warnings remain in place for Diamond Harbour, Purau Bay, Church Bay, and Charteris Bay while authorities continue to monitor water quality.