The Prime Minister supports adding fluoride to water but says the Government won't force councils to fluoridate.
Last week, Hamilton City Council voted to stop fluoridating its water supply, a move strongly criticised by health experts.
Speaking at the Fieldays event at Mystery Creek near Hamilton on Friday, John Key said he personally supports fluoridation.
"It's a matter for local councils, and not every community in New Zealand has fluoride. But in my view the evidence strongly supports that fluoride actually stops tooth decay.
"Overall, the health benefits of it are significant, and actually those that worry about the negative health effects overstate those matters, in my view."
Local Government New Zealand and the Dental Association say councils should never have been given the power to choose whether to fluoridate water.