Christchurch's water will be chlorinated.

Christchurch City Council voted on Thursday for the chlorination of the city's supply, which will take about 60 days to complete.

It was decided to do it after assessment of the below-ground wells found they were not sufficiently sealed to eliminate the risk of contaminated surface water seeping into them.

The Canterbury Water Assessor removed the city's secure bore status.

The contamination of Havelock North's water supply in 2016, and subsequent government enquiry, meant chlorination in Christchurch was an unwanted but necessary measure to take, head of the council's three waters, John Mackie, told councillors.

The government warned that hundreds of thousands of New Zealanders were at risk of getting sick and universal treatment of drinking water was needed.

Mr Mackie said "a range of work" would be taken on 103 boreheads, costing $800,000, and it was hoped the chlorination could be halted in 12 months.

"The quality of drinking water has not changed, we still have a rigorous regime in place to test," he said.

Canterbury Medical Officer of Health, Alistair Humphrey, said he had been satisfied with the city's water for 18 years, but chlorination was needed to meet the government's new standards.

"We will not need to always chlorinate, but these are a special set of circumstances," he said.

"There is no need to boil water and there is no immediate risk."

Councillors had their say in a lengthy debate.

"No one wants to be in this situation, we treasure our precious water, but to oppose (chlorination) would not be responsible," councillor Yani Johnason said.

"We are bring proactive before we have a public health crisis," councillor Glenn Livingstone said.
Mayor Lianne Dalziel said it was a temporary solution.

"The risk (of contamination) are very small, but the consequences are huge. Since Havelock North the emphasis has been on secure water supply and it's not a risk I'm prepared to take."

The vote to chlorinate wasn't unanimous, with three councillors - Aaron Keown, Mike Davidson and Sara Templeton - against.

Ms Templeton said there had not been enough community engagement before the decision was made.