The New Zealand Supreme Court Building in Wellington. The court has ruled in favour of South Taranaki District Council in its legal battle to add fluoride to the water supply in two towns.

A Taranaki council has won a landmark legal fight to add fluoride to the water supply in two towns, and the Supreme Court decision has been praised by New Zealand's dentists.

On Wednesday the court ruled against anti-fluoride campaigners following a five-and-a-half-year, $385,000 legal battle over the water supply in Patea and Waverley.

South Taranaki District Council decided to add fluoride in December 2012.

Catherine Groenestein/STUFF South Taranaki Mayor Ross Dunlop said he was very pleased to have a definitive ruling.

The decision, which followed public consultation, was taken to improve poor dental health in both towns, but was challenged by the group New Health New Zealand Incorporated by way of a judicial review in the High Court.

New Health New Zealand, based in Christchurch, argued that fluoridation was a medicine and therefore outside the council's powers. It also argued that people had a right to refuse treatment under Section 11 of the New Zealand Bill of Rights Act.

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The group lost its initial challenge and an appeal to the Court of Appeal was dismissed, but in October 2016 it was given the right of appeal to the Supreme Court.

The council argued that it had the authority to introduce fluoride, which it said was not a medicine. It also argued that even if fluoride did amount to a medicine the law was subject to "reasonable limits".

A majority of Supreme Court judges have now ruled that the council has the legal authority to fluoridate water and this is not constrained by the Bill of Rights Act.

South Taranaki District Council has spent $45,000 fighting the case, with the Ministry of Health and District Health Boards contributing $340,000.

In a statement, council mayor Ross Dunlop said he was very pleased to have a definitive ruling after five and a half years.

"Since council's decision in December 2012, New Health have appealed to the High Court, Court of Appeal and Supreme Court, all of which have been comprehensively dismissed.

"Now we need to take some time to properly digest the Court's 113 page ruling before making any decisions about implementation."

The decision was also welcomed by the New Zealand Dental Association.

"We have seen South Taranaki District Council take part in expensive and unnecessary legal proceedings, at a cost to ratepayers. A cost that has also fallen on the Crown via the Attorney General," Dr David Crum, CEO, said in a statement.

"Community water fluoridation is effective, safe and affordable, we will soon see decision-making for this sit with DHBs. This means that no council will have to face what South Taranaki District Council did as a result of making public health decisions for their community.

"The Centers for Disease Control and Prevention lists water fluoridation as one of the ten great public health achievements of the 20th century."

Water industry body Water New Zealand called the verdict a "victory for common sense and evidence-based decision making".

Its chief executive, John Pfahlert, said in 2014 the Royal Society of NZ and the Prime Minister's Chief Science Advisor concluded there was no evidence that fluoride had adverse effects on public health and had significant dental health benefits.

Mr Pfahlert said councils could now act to fluoridate public water supplies in the knowledge that there is both a sound legal and scientific justification for doing so.

New Health New Zealand was unavailable for comment.