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The Selwyn District Council added a second low-dose of chlorine to the Dunsandel supply on January 15. It had planned to remove the chlorine after a week but this was not possible.
Council infrastructure service delivery manager Gareth Morgan said the round of chlorine treatment is now expected to continue until mid-February.
"We have identified the cause of the coliforms in the water supply at Dunsandel and we’re carrying out repairs to fix this," he said.
"While we’re doing these repairs we’re putting a temporary reservoir in place and that water is being treated with low-dose chlorination to ensure it is safe."
The first chlorine treatment was added to the water supply on January 7-10 and led to a significant reduction in the number of coliforms.
This council said the current low-dose chlorine treatment is precautionary only, and no E. coli has been detected within the water supply.
Residents do not need to boil their water.
The Dunsandel supply provides water to about 200 households.