A majority of the judges in New Zealand's highest court have upheld the South Taranaki District Council's right to fluoridate water supplies in Patea and Waverley.
The council has been in a long-running legal battle with anti-fluoridation campaigners New Health New Zealand since its 2012 decision to fluoridate the water.
The group has fought the council's decision through the courts beginning with a judicial review in the High Court.
The High Court ruled in favour of the council and that was upheld by the Court of Appeal.
Both those courts ruled that the South Taranaki District Council had statutory authority to fluoridate the Patea and Waverley water supplies, and that fluoridation was not a medical treatment.
They also found that if fluoridating the water engaged the Bill of Rights right to refuse medical treatment, fluoridation of water was a justified limit on that right.
A second appeal was also before the Supreme Court relating to whether the two compounds added to water supplies for fluoridation, hydrofluorosilicic acid (HFA) and sodium silicofluoride (SSF), were medicines in terms of the Medicines Act 1981.
At a hearing last year the council's lawyer told the Supreme Court medical evidence showed there was a high level of tooth decay in Waverley and Patea, and fluoridating the water supply was a proportionate way of dealing with that.
However, New Health New Zealand said fluoridating town water supplies breached residents' right to refuse medical treatment.
In today's decision four out of the five Supreme Court judges have dismissed New Health New Zealand's appeal, ruling Taranaki District Council can fluoridate local water supplies, as it has a duty under the Health Act to protect and improve public health in its region.
Justices William Young, Glazebrook, O'Regan and Ellen France found that power was not constrained by the Bill of Rights.
"On our approach, that is because the authorising provisions limit the ... right [to refuse medical treatment] only to an extent that is demonstrably justified in a free and democratic society."
In her part of the decision Justice Glazebrook said, "I accept that dental decay is a condition that a local authority would be entitled to consider injurious to public health."
"Thus, there would be a specific power under ... the Health Act to 'cause all proper steps to be taken to secure … the removal of the condition'."
Justice William Young accepted people living in areas with fluoridated water would find it difficult to avoid drinking it.
However, he had reservations over whether that meant legally those people were being denied the right to refuse to drink it.
Regarding the question of whether fluoridation involved medical treatment, he said the more routine an activity was, the less likely it was to be thought to involve medical treatment.
"It would be odd [for example] to regard a parent who rubs sunscreen onto a child or brushes that child's teeth as providing medical treatment."
"It has never been suggested that the supply of naturally fluoridated water involves medical treatment. I do not see why the supply of fluoridated water which is materially the same in chemical composition as naturally fluoridated water should be regarded differently."
However in a dissenting judgement, Chief Justice Dame Sian Elias ruled there was no basis to interpret the Local Government or Health Act as giving authority for local bodies to add fluoride to water.
"The power relied upon in supporting an implied [ability] to add fluoride to water supplied by the council is contained in a [section of the Health Act] which authorises a local authority to take 'all proper steps … to secure the abatement' of any 'nuisance' or any 'condition likely to be injurious to health' ... in the district," Justice Elias said.
"Lack of fluoride cannot be described as a 'nuisance', a term defined ... by reference to conditions likely to be injurious to health, such as through accumulation of rubbish or through the condition of drains or watercourses."
Justice Elias said if Parliament wanted to empower local authorities to add fluoride to reticulated water for public health purposes, it could do so.
She said she would therefore have allowed New Health New Zealand's appeal and made a declaration that the council has no power to add fluoride to the water it supplies.
The Supreme Court unanimously dismissed New Health New Zealand's appeal regarding changes to the Medicine Act, which declared fluoridating agents were not medicines.
It also ordered the organisation must pay $20,000 costs to the South Taranaki District Council.