The cost of keeping Christchurch's drinking water safe has spiralled to almost $40 million – more than 25 times the city council's initial estimate for chlorination and to fix vulnerable wells.

Christchurch City councillors have to find at least $35m to fix 49 pump stations, more than 40 times the original $840,000 budget for the work.

In addition, the controversial chlorination programme – which the council was told in January would cost $690,000 – has rocketed to $2.25m for the supply and installation alone.

Another $1.3m is budgeted for a year-long maintenance contract, beginning next month, bringing the cost of chlorination to more than $3.5m – five times the original estimate.

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The budget blow-out was revealed to councillors in a report on Thursday by strategy expert Helen Beaumont, who was parachuted in last week in a bid to get the city's beleaguered drinking water improvements on track.

Money for the programme is likely to only be found by dipping into budgets reserved for years down the line.

SUPPLIED Christchurch residents have made their frustrations clear about the smell and taste of chlorine in their water.

The report appears to confirm that little work to repair any of the well heads at the city's 49 vulnerable pump stations has yet been carried out.

A $5m, 12-month project will repair seven pump stations, raising well heads and repairing those already above ground.

But it is not close to starting – work is out to tender for two of those pump stations and in the design stage for the remainder.

Council staff are still investigating the best ways to fix the remaining 42 compromised pump stations.

Options include raising well heads, disinfection with UV or ozone treatment, drilling replacement wells or turning some wells off entirely in a bid to bring the supply up to drinking water standards.

UV treatment could be used at 12 pump stations and ozone treatment at one more as an alternative to chlorination.

But costs for UV supply, installation and work to run the programme range from $21.6m to $86.4m, depending on the size of the pump stations.

Ozone treatment could cost between $5m and $8m, while an option to drill new wells would likely come with a price tag of $1m each.

DAVID WALKER/STUFF People have turned to a small, local spring for their water rather than drinking from the city's chlorinated supply.

The council's draft long-term plan (LTP) includes a $35m budget over 10 years for work to repair wells.

In the report, Beaumont said: "This could be brought forward into the first three years of the LTP, with any additional funding that may be required being consulted on as part of a future annual plan, LTP, or special consultative procedure.

"It is likely that implementation of the full work programme would exceed $35m."

Following a public backlash, efforts to reduce the impact of chlorine are continuing, with pipes flushed to rid them of organic matter that causes the smell and taste, reduction of doses and favouring non-chlorinated pump stations to increase the amount of untreated water in the supply.

Despite this, Mayor Lianne Dalziel remains adamant the water supply will be chlorine-free within the promised 12 months.

Councillors signed off on a year of chlorination in January. Treatment started on March 26 and the last pump to be chlorinated was on May 9.

Chlorination beyond 12 months would require a council vote.

IAIN McGREGOR/STUFF Mayor Lianne Dalziel is determined the city's water supply will be chlorine-free within the mooted 12-month timeframe.

"There is a misconception that every well head needs to be brought above ground in order to stop chlorination," Dalziel said.

"That is not the case and I remain determined to have the council resolve the chlorination issue within the 12 months."

She said she would "fight tooth and nail to have unchlorinated water" restored.

"We all know this is part of who we are as a city," Dalziel said.

"I just want to – come hell or high water – commit to this 12 months."

Christchurch's water problems, she said, are not comparable to the situation in Havelock North, where in August 2016 more than 5000 people fell ill with gastroenteritis due to E. coli in the water supply.

"In Havelock North the source of the contamination was most likely the pond that was then able to lead to the bore, which was part of an unconfined aquifer," she said.

"We are talking about our confined aquifers, it is a completely different system."

Beaumont said the goal of the programme was to ensure Christchurch's water supply was "safe and good to drink".

"You've acknowledged that there has been considerable debate about whether or not we can achieve this within the 12 month period," she said.

"That does remain the target, however it is a challenge."

The report also hinted at the possibility of water restrictions in coming months to keep water use at winter levels – half those of the summer peak – to allow wells to be turned off for works to take place.

Beaumont said: "We will be talking about a water conservation programme."

"Success depends in part on the resources we can bring to the programme and accelerate the work programme and it depends in part on the people of Christchurch cooperating with us in a probable water conservation programme.

"In order to be able to deliver water to the city, to meet the demand for the city, and carry out the repairs we have to take a number of wells offline.

"In Christchurch our winter demand is approximately half of our summer demand, so if we can hold the demand closer to winter levels than summer levels right throughout the year we have a much greater chance of delivering this programme."

Meanwhile, some councillors, including Phil Clearwater, defended the council's progress to date.

"The sadness really has been the focus by the media on the failures ... a helluva lot of great work is being done to rectify a situation, which wasn't really our making at all," Clearwater said.

Councillor Glenn Livingstone said ratepayers had a "thirst" for information, and suggested the council provide as much information as possible through its own "news" service.

"As much information as we can get out to Newsline I think as constantly as possible would be very useful."