A water-system failure that is probably the cause of more than 100 Darfield residents being struck down by gastroenteritis is being investigated by the Selwyn District Council.

The announcement comes after Canterbury District Health Board medical officer of health Alistair Humphrey said the council needed to "wake up to its responsibility" and address its recurring water issues.

There have been 22 confirmed cases of campylobacter, a nasty bacteria that causes vomiting and diarrhoea, and more than 100 gastroenteritis notifications. E. coli was found in the town's drinking-water supply and a boil-water notice was issued on August 17.

It was lifted four days later, but some residents did not receive letters telling them to boil water until after the notice was lifted.

Humphrey said more than one barrier had failed, resulting in the "serious outbreak".

"Intensified farming now means that a lot of water is contaminated with animal faeces, especially the Waimakariri after heavy rains, and secondly, the chlorination of this heavily contaminated water fell short of levels required to make it safe."

Darfield was the largest town in the country to still rely on septic tanks and boulder holes, with no reticulated sewerage system, he said.

The region's lack of a reticulated sewerage system did not lead to the outbreak but was "an ongoing contamination risk".

The CDHB made a submission to the council's 2010-22 long-term plan asking for upgrades to the region's water systems to be a priority but was told there was "currently no justification".

Selwyn chief executive Paul Davey said the council was "very mindful of its responsibilities" and was "constantly looking at ways" to improve water quality.

"We are viewing this failure very seriously and we are undertaking both internal and external peer reviews of the incident to understand how it happened so we can mitigate future events."

The council has issued eight boil-water notices since September 2010.

Davey said a reticulated sewerage system would cost about $10,000 for each household.

"There's no evidence that there's a public health risk through the perfectly functioning septic tank regime or even any call for a reticulated sewer scheme in Darfield," he said.

Darfield's deep-water well had a pump failure in June and the supply reverted to the Waimakariri River while it was being repaired.

The council is reviewing its communications.

Some residents did not hear about the boil-water notice in time.

- © Fairfax NZ News

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