A Whangarei dentist says although there is no hard proof fluoride is good for growing teeth, international studies show it can reduce children's intelligence.

Anti-fluoridation campaigner Laurie Brett said claims fluoride makes teeth stronger and prevents decay are old fashioned, unscientific and not backed up by evidence.

Dr Neil Croucher, Northland District Health Board oral health adviser, disagrees.

Northland has consistently had one of the country's highest tooth decay rates, especially among children and adolescents with significant ethnic (Maori and non-Maori) oral health inequalities, Dr Croucher said.

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The high rate of tooth decay affects general health and well-being and results in significant costs to the health sector – yet, much of this is potentially preventable, he said.

The addition of fluoride to water supplies brings New Zealand's low natural level of the mineral in soil and water to an optimum level that prevents tooth decay, Dr Croucher said.

The Ministry of Health claims fluoride helps prevent decay by strengthening the tooth surface, reducing the growth of bacteria and helping repair early-stage decay.

The Northland DHB considers community water fluoridation a cornerstone of prevention and among the top 10 best population-based public health measures in reducing tooth decay, Dr Croucher said.

Northland remains non-fluoridated after a brief foray into reticulated fluoridation in Kaitaia and Kaikohe was abandoned in 2009.

The anti-fluoride argument cites a twice published and reviewed Harvard University analysis, funded by the National Institutes of Health (NIH), concluded that children who live in highly fluoridated water areas have on average 5 per cent lower IQ scores than those in low fluoride areas, Mr Brett said.

Studies in China, India, Iran and other countries had similar results, and also linked fluoride with ADHD and learning difficulties.

Dr Connett is a retired professor of environmental toxicology and chemistry at St Lawrence University. He is on a New Zealand speaking tour as the guest of Fluoride Free NZ.

Mr Brett said programmes in fluoride-free Denmark, Scotland and other countries where governments strongly funded public educational and dentistry programmes have seen tooth decay rates plummet.

The programmes largely involve giving children toothpaste and brushes, dietary and other health information, and getting parents on board.

In last year's budget, the National Government introduced legislation placing the power to put fluoride in water supplies in the hands of district health boards, without local consultation or mandate.

As the task of physically adding fluoride to water supplies falls on district councils, the Government also pledged up to $12 million to help them cover the costs. The legislation is expected to come into play this year.

Mr Brett said people who objected to fluoridation should contact their local MP, their district council and district health board.

About half New Zealand's population has fluoride in the water supply, although that figure is greatly boosted by Auckland where the majority of public supplies are fluoridated.

''But as we know, less than 1 per cent of all reticulated water in New Zealand is actually drunk, so where does all the fluoride end up?'' Mr Brett said.