Fluoridation of Canterbury's drinking water is shaping up as a major issue at at October's district health board elections.
The government has proposed handing the decision to fluoridate to DHBs, forcing candidates to take a position on a contentious topic.
With a population of more than 500,000, Canterbury is by far the region with the largest number of people drinking unfluoridated water.
RNZ spoke to all seven elected board members but only two were willing to offer an opinion on fluoride; David Morrell who supports it and Andy Dickerson who is against it.
Fluoride Free New Zealand national co-ordinator Mary Byrne said her group was standing its own candidate in the DHB elections and would actively promote those against it.
Not enough was known about the impact fluoride could have on mental health and it did not make any difference in reducing rates of tooth decay, she said.
"People in Canterbury haven't been worried about fluoridation up until now and we now know there are a lot of people concerned because we were at an expo there a couple of weeks ago and the vast majority of people knew about fluoridation potentially coming and they were definitely opposed."
But the clinical director of the region's community dental service, Martin Lee, said Ms Byrne and her group were wrong.
"Fluoridation first started in New Zealand in Hastings in 1954 and they've been making claims that fluoride causes defects in brain function, cancer, the list goes on and on and on - and this has been going on for over 60 years.
"Now you've got to figure, if there was something to it, we would know by now."
The benefits of fluoridation in combination with regular brushing and a low-sugar diet were clear, and groups like Fluoride Free New Zealand were spreading myths rather than facts, which was frustrating, Dr Lee said.
"It is really, really easy to plant the seed of doubt and it is very hard to root out once you've got it in there," he said.
"The anti-fluoridationists become very vociferous and they claim they've got piles of highly placed experts who are saying fluoridation is bad for you or it doesn't work, and it's not true."
The legislation passing responsibility for putting fluoride in the water to DHBs is still being drafted.
It's expected any change would not take effect until at least 2018.