Nearly 30,000 people in the north-west of the city are now drinking unchlorinated water.
Remedial works have been completed and chlorination at the Burnside and Farrington pump stations has now stopped.
But city council water supply improvements manager Helen Beaumont said it would only be possible to keep these stations chlorine-free during low water demand – such as winter.
“Where the demand exceeds the amount of water, we can deliver from secure wells then the bores with below ground well heads will need to be brought back into service and chlorination would resume at that pump station,” Ms Beaumont said.
She said the high demand period for water is generally November to the end of March. But a water conservation programme may run in a bid to continue minimising the need for chlorination where possible.
The city council is working through the same process at a further five pump stations, to stop chlorination in further parts of the north-west, central and Ferrymead zones.
That would mean another 52,0000 people will have untreated water this month, affecting 17,500 households in the north-west, 6000 in the north-east and 3700 in Woolston.
“Once the chlorination has ceased at the seven pump stations where the below ground well heads can be isolated, the proportion of water without chlorination will increase from approximately 5 per cent to approximately 24 per cent,” said Ms Beaumount.
Three pump stations in Prestons and New Brighton, servicing 7500 households, avoided chlorination altogether.