The Christchurch City Council has decided the city's water will be chlorinated for the next year.
Earlier this week the Christchurch City revealed engineers concerns that some of the below ground wellheads could become contaminated, especially during heavy flooding.
Council staff have recommended that temporary chlorination be installed at all of the city's 56 pump stations, at an initial cost of $600,000.
The damaged wellheads will also be repaired.
Today the council voted 12-3 in favour of chlorinating the water supply.
Christchurch Mayor Lianne Dalziel said even though the risk of contamination was low, there would be huge consequences if there was a problem.
Ms Dalziel said if there was a water crisis like the gastro outbreak in Havelock north in 2016, which landed dozens in hospital, hundreds of thousands of Christchurch residents could be infected.
Most councillors reluctantly supported the move.
"It's the last thing I ever thought we would agree too," said councillor Pauline Cotter.
"I would love not to, but as someone who has a young daughter, we need to do the responsible thing," councillor Yani Johanson said.
"It's our responsibility to provide safe drinking water, especially to the [city's] vulnerable," councillor Anne Galloway said.
However, some councillors opposed the move to chlorinate.
"I totally oppose adding chemicals to our pristine water," said councillor Mike Davidson, who received a round of applause from some members of the public.
"Our water is perfect, it has been through a lot, but it is still perfect."
Councillor Sara Templeton wanted to delay a decision until the council could consult the public on the issue.
Councillor Aaron Keown also opposed chlorination.
It will take 60 days to set up the chlorination systems.
After the meeting, Ms Dalziel said nobody wanted to see Christchurch's pristine water chlorinated.
"But when we are faced with a situation where we can no longer rely on the secure status of our groundwater because of the standard of the bore heads, and the fact that the engineers won't sign off on them, what else can we do as a council?"
'You're talking about mass-dosing people' - Gerry Brownlee
Ilam MP Gerry Brownlee said Christchurch was well-known for having high-quality drinking water, which did not smell or taste of chlorine.
A decision to chlorinate it should not be made lightly, he said.
"You're talking about mass-dosing people with chlorine. It is a poison, that's why it's in the water to kill off all the bugs."
However, Infrastructure group manager Ashley Harper yesterday said the gastro outbreak in Havelock North in 2016, which left dozens in hospital, changed people's minds.
"We are recommending that we take steps to ensure that doesn't happen in our district," he said.
In December last year, a report into the Havelock North gastro outbreak warned hundreds of thousands of New Zealanders - one in five people - were at risk of getting sick and universal treatment of drinking water was needed.