Christchurch City Council has decided against temporarily chlorinating the water in parts of the city's northwest, while it decommissions unsafe shallow aquifers.

Chlorinating water in Christchurch's northwest is off the table, for now.

The Christchurch City Council went against its own staff advice and unanimously decided on Thursday not to consider temporarily chlorinating the water from eight shallow wells that feed into three pump stations, serving about 20,000 residents.

The council instead decided to accelerate a $16 million programme to replace 22 shallow bores, supplying 80,000 northwest households.

The work was originally due to be finished by June 30, 2018, but most of the wells would now be decommissioned by March 2017. Fourteen of the most vulnerable shallow wells have already either been decommissioned or shut down.

READ MORE:

* Water in Christchurch's northwest could be chlorinated

* 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

Accelerating the work would cost an additional $480,000.

The council would also embark on a programme to raise community awareness of the risks of drinking untreated water from the shallow bores.

Canterbury's medical officer of health, Alistair Humphrey, last month asked the council to explain why its continued use of the shallow wells did not present "an untenable risk". Humphrey's request was prompted by a gastro outbreak caused by campylobacter in the water supplying the town of Havelock North in Hawke's Bay.

Staff will now talk to Humphrey to see if he was satisfied with the council's response, without chlorinating the water. They will report back to the council in November.

Water from the bores was tested for E.coli daily, but it took at least 24 hours to get the results, so there was always a 24-hour period where contamination could go undetected, council three waters and waste boss John Mackie said.

He said the council complied with the water standards, but his professional advice to the council was to chlorinate the water, which would eliminate the risk.

Mayor Lianne Dalziel asked Mackie if the risk from the shallow bores had changed in the last few years. He said no.

She said it was only the perception of risk that had been heightened since the Havelock North contamination.

Deputy Mayor Vicki Buck asked how long had the city been using the shallow bores.

Mackie was not able to say exactly how many years, but said it was a "very, very long time".

​Cr Pauline Cotter wanted to know how much it would cost the council to provide people, who were concerned about the risk, access to other drinking water.

Council city services general manager David Adamson said most people could get water from a friend's house in another part of the city, if they did not want to drink the water.

The shallow wells were less than 43 metres deep and some were 16m deep. They would be replaced with wells that were in excess of 100m deep.

The council had been considering to temporarily chlorinate the water going through the Avonhead, Farrington and Grampian pump stations. This would cost $10,000 to set up and $15,500 a month to maintain.

The most vulnerable shallow wells at the Burnside, Harewood and Wrights pump stations have been shut down, but could be used during times of high demand.

Water from the shallow wells had a "D" risk grading, which was classed as an unsatisfactory level of risk. The rest of Christchurch's water supply had a "B" grading, which represented a satisfactory level of risk.

E. coli bacteria has been found in Christchurch's drinking water 125 times in the past four years. Sampling data from the 14 positive tests in the past year suggested water supply reservoir tanks were the main contributors to the positive tests.