Northland Health is claiming a small improvement in children's teeth after two years of fluoridated water was introduced to Kaikohe and Kaitaia.
But Lawrie Brett, who opposes fluoridation, says the study is unreliable due to its small sample size and poor controls.
Dr Brett says it also found a small increase in some decay rates in Kaikohe and a sharp drop in decay in Moerewa, which is not fluoridated.
The Far North District Council plans to hold a referendum on fluoridation next year.