Adding fluoride to Christchurch’s drinking water could cost the city about $10 million, according to a preliminary estimate.

It has some questioning whether the health benefits of fluoridation outweigh the cost.

Councils decide whether to fluoridate their public drinking-water supplies, but a bill making its way through Parliament would hand that decision-making to District Health Boards (DHBs) – although councils would still have to foot the bill.

The bill passed its first reading and is now open for public comment.

Information requested from Christchurch City Council staff by councillor Vicki Buck estimated the cost of fluoridation to be around $10.5 million, with an ongoing cost of about $500,000 per year.

The estimate, which was “very preliminary”, was based on the cost to set up dosing sites around the city.

Buck said it was clear fluoridation would be expensive, but under the proposed legislation council may not get a say on whether it happens.

“It’s unusual for the authority that’s not responsible for implementing it to make the decision,” she said.

“I just like the idea that whoever makes the decision is responsible for the implementation as well, not somebody telling you you have to do it.”

Fluoridation had not been on the council’s agenda at any point in the last 15 years, she said.

Mayor Lianne Dalziel said last year she opposed fluoridation due to the likely cost. Her predecessor, Sir Bob Parker, was also against.

The Canterbury DHB (CDHB) has long supported fluoridation, signing a position statement in support in 2003.

Christchurch is New Zealand’s largest city without a fluoridated drinking water supply.

The city’s water is entirely untreated, due to its high-quality source: water that falls as rain high in the Southern Alps, which flows down the Waimakariri River and seeps into aquifers beneath the city.

Around 170 wells draw from the aquifers, with about 55 council pumping stations.

The complex system, combined with the lack of existing treatment infrastructure, meant any additions to the water would be costly.

Canterbury Medical Officer of Health Dr Alistair Humphrey said the costs of fluoridation would have to be carefully weighed against the benefits, which were significant.

“The situation in Christchurch is different to many parts of the country because we do have well over 100 separate bores providing drinking water to the population, which means treatment of the water supply is complicated,” he said.

“Christchurch is a special case where we will need to consider the issue very carefully.”

There was no doubt fluoride was an effective way to prevent tooth decay, a preventable disease that tied up valuable hospital resources, he said.

“FTooth decay is one of our most serious non-communicable diseases.

“It ties up a lot of theatre time for our district health boards … because children often require general anaesthetic to remove teeth.

“The cost goes beyond just the tragedy of a child whose diet and lifestyle is affected by dental caries, it also impinges on the ability of the health service to provide other important surgical operations.”

Research in 2004 found Canterbury children had significantly higher rates of tooth decay than those in Wellington, where water is fluoridated.