DIVIDING OPINION: This billboard on Ulster Street and another one like it in Mill Street are wrong and potentially defamatory, the New Zealand Dental Association says.

Anti-fluoride billboards erected in central Hamilton are misleading and possibly defamatory, the New Zealand Dental Association says.

But one of the dentists pictured in the Fluoride Action Network New Zealand billboards in Ulster and Mill streets has hit back, saying the criticisms are reflective of a "police state".

The signs declare "informed dentists say no to fluoridation".

Hamilton dentist and association member Steven Pawley said the billboards were insulting, false and bordering on defamation.

"The fact FANNZ have had to use dentists from outside the Waikato region is indicative of how hard it is to find dentists who do not agree with the safety and efficacy of community water fluoridation," Dr Pawley said.

"These three dentists have an opinion and they're entitled to it, but they are a very small minority of our professional group and certainly do not represent the NZDA's position.

"Our unequivocal position is that fluoride in water is imperative to oral health. It is safe. It is cost-effective and fair for the entire population - especially our children who benefit most."

The New Zealand Dental Association has more than 2000 members.

Dr Pawley also said the billboards go against the Dental Council of New Zealand's code of practice on advertising, which comes into effect on November 1 and states: "If your personal belief differs from the majority view, then there is an onus on you to inform your audience on the extent to which your view has, or does not have, the support of the majority of oral health practitioners".

He urged Hamilton residents to join their local dentists in voting for fluoride being returned to the water supply in the upcoming referendum, which is being run in conjunction with local body elections in October.

Northland dentist Laurie Brett, one of the three dentists pictured on the billboards, said he believed Dr Pawley's criticisms were unjust.

"No professional body really can override an individual's opinions," he said. "It's welcome to the police state.

"The knowledge base on fluoride is evolving all the time towards cessation. The whole nation of Israel just stopped fluoridating. The bottom line is that it's old school thinking that needs to get kicked into touch."

For more information visit www.waikatodhb.health.nz/fluoride or, for the alternative view, go to www.fannz.org.nz.