Christchurch drinking water could be temporarily chlorinated while below-ground wells at risk of contamination are sorted.

Christchurch's drinking water should be temporarily chlorinated after it was found the city's wells may be in danger of contamination, Canterbury's public health boss recommended.

An assessment of underground well heads found some with cracked surrounds and inadequate seals, meaning they may be vulnerable to pollution from surface groundwater.

The Christchurch City Council has known about the risk since December 22 when the Canterbury Water Assessor removed the city's secure bore status, but only made the information public on Tuesday.

IAIN McGREGOR/STUFF Canterbury medical officer of health Alistair Humphrey takes a sip of water during a media briefing on the contamination risk to Christchurch's public water supply. In the background are Christchurch Mayor Lianne Dalziel and city council chief executive Karleen Edwards.

Mayor Lianne Dalziel defended the month-long wait for residents to know of the potential harm, saying the council only heard about it at 3pm on the last working day before Christmas and it "wasn't brought to high enough attention in the organisation".

READ MORE:

* Chlorinating water in Christchurch's northwest is off the table

* Health officials want assurance D-rated northwest Christchurch water supply safe

* Christchurch's drinking water contaminated 125 times in four years

* Positive E coli tests 'not surprising' in Christchurch untreated water supply

* Chlorination extends to Hastings after E coli found in tanker with 'clean' water

* City council shuts down contaminated Wrights Rd water bore

* Havelock North: a village that looks as ghostly as its residents

Dalziel said she was advised of the situation on January 15 when she returned to work after the Christmas break.

IAIN McGREGOR/STUFF Mayor Lianne Dalziel says the risk to public health has not changed and the public water supply is safe to drink.

Chief executive Karleen Edwards said council officers had "not been idle" in the interim, but had worked with health officials to understand the implications and how best to secure the wells.

Officials were at pains to emphasise the extremely low risk to public health and that drinking water remains safe.

Canterbury medical officer of health Dr Alistair Humphrey said the public would have been warned to boil their water if the danger warranted it, but such advice was unnecessary because there was no risk of imminent water contamination.

STUFF Christchurch's drinking water is untreated, but has lost its "secure status" after engineers assessed some of the city's below-ground wells.

But he believes drinking water should be temporarily treated while well heads are fixed – a decision that will be voted on by councillors on Thursday.

"My advice is, in conjunction with the engineering staff at Christchurch City Council, that they should temporarily chlorinate the water for the safety of our community until such time as the engineers once again guarantee the secure groundwater status."

Councillors were made aware of the issue during a meeting held in secret last week.

COUNCILLORS TO KEEP OPEN MIND

Cr Mike Davidson said he would wait until Thursday's meeting to decide whether he favoured temporary chlorination.

"The drinking water in Christchurch is exactly the same as it has been for some time.

"Does the changing of one word from provisional to unsecured mean we have to chlorinate our water until we get that secured status back? Because the risk hasn't actually changed."

He was disappointed it took nearly four weeks to be told about the changed water status.

Cr Pauline Cotter said she would keep "an open mind" about chlorinating the water.

"Personally, from my heart, I do not want to put anything in our beautiful water. At the same time I don't want to be responsible for a possible crisis and people being sick and even dying."

She understood the delay in making the information public was "inadvertent, it was not deliberately withheld".

"The 22nd of December was a Friday . . . I think the email arrived? I'm not sure who it went to, either the [chief executive] or the manager of waterways . . . about 3.30pm-4pm on Friday.

"That hit council pretty much on Christmas Eve. That's the question on my mind – why did it arrive at that time?

"We've been aware of it now for less than a week and we've acted on it in the most transparent way we can."

Cr Aaron Keown would not say how he would vote, but said "given it's only temporary . . . it's not that big a deal".

"My main thing is whatever the outcome is here we get Christchurch to a situation were we don't ever need chlorination of our water."

Cr David East supported Humphrey's recommendation, adding it was likely the council would accelerate its programme to repair underground well heads.

"At worse it would be a few months and we hope it can be significantly less than that."

Cr Glenn Livingstone said he would take an open, but "better safe than sorry", approach to Thursday's meeting.

"There's a strong chance I would lean towards it if we have to chlorinate," he said.

Cr Tim Scandrett said chlorination already happened when bore heads were cleaned or changed.

"I think we've got really good water quality, we've just got to make sure we maintain that," he said.

Cr Phil Clearwater said the risk was "too great to do nothing".

"With some reluctance I support the need to have temporary chlorination."

Cr Jamie Gough said chlorination should be seen as a "last resort", but it would be "foolish to sit on your high horse with your ideals on water at the expense of the public's health and safety".

He said there were "initial murmurings about potential chlorination" pre-Christmas, but he was unaware of any formal briefings.

"I think it would have been nice for people to know earlier, but as far as that having a material impact on anyone's life it seems to be neglible."

WELL WORK 'FAST TRACKED'

Humphrey said there was no option but to chlorinate. Asked his opinion if the vote was not passed, he said: "I would be concerned."

Assuming the move is approved, chlorination at the city's 56 pumping stations would likely take about two months.

John Mackie, the council's head of three waters and waste, said water was being tested every 24 hours, above and beyond required standards.

He estimated the cost of fixing the well heads and any chlorination programme was likely to be about $1.4 million.

The issue came to light after the council's water supply contractor, Citycare, undertook a detailed study of well heads following the first report into the Havelock North crisis, which left more than 5500 people ill and was linked to several deaths following an outbreak of bacterial infection in 2016.

The assessment, which began in June, found some of Christchurch's 103 wells that are below ground may not be sufficiently sealed to prevent contamination from surface groundwater, particularly after heavy rain.

Work has already been carried out to fix seven of the most vulnerable, and the rest are expected to be secured by the end of the year.

Dalziel said the city took "extraordinary measures" to ensure the safety of the drinking water supply and that there was no impact on groundwater.

Mackie added: "There is no current issue with the quality of our groundwater – it is safe to drink.

"The issue we face is how we can eliminate the slight risk of contamination occurring through our infrastructure."

The discovery of an issue with well heads does not automatically mean there is an increased risk to drinking water quality, and testing before it was identified found no problems with drinking water quality.

Humphrey said that while the risk had not necessarily increased, the consequences of any contamination were grave, and compared it to risk management in the airline industry.

"The probability of it occurring is very small but the consequences of contaminated water in Christchurch are quite serious.

"In Havelock they had 5000 cases [and] three, possibly as many as five deaths associated with the water contamination.

"Here, if we had a similar thing happening in a city, it might not be 5000 – it might be as many as 15,000 or 20,000, with a corresponding increased number of people seriously ill or indeed dying.

"We have to have that rigour in terms of safety because of the risk, not because of the probability."