Christchurch's water supply will be temporarily chlorinated, the city's councillors have decided.
Councillors voted to temporarily chlorinate the city's water supply when they met in the council chambers on Thursday.
This vote follows Tuesday's recommendation by Canterbury medical health officer Dr Alistair Humphrey to temporary chlorinate the water.
Engineering assessments revealed some of the city's wells were not sufficiently sealed and may be susceptible to contamination.
The groundwater wells in Christchurch are no longer deemed to be secure due to the slight risk of contamination in significant rain events.
Mayor Lianne Dalziel said on Thursday the risk of contamination is "unlikely" and "small" but the consequences would be "huge".
If the water were to become contaminated, hundreds of thousands of people would be exposed, Dalziel said.
Christchurch's drinking water is drawn from 156 wells located at 56 sites across the city.
All 103 below-ground wellheads need to be upgraded.
The water won't be chlorinated for up to another two months as treatment infrastructure is put in place.
The temporary treatment is expected to cost around $800,000 and is due to be completed by December this year.
The mayor and councillors voted 14 to three to temporarily chlorinate the water.