Anti-fluoride campaigners are alarmed at a "sinister" move to slide through a law change over Christmas, validating councils' power to add the chemical to drinking water.

Medsafe opened public submissions on November 25 on a law change to stop fluoride being legally defined as a medicine - meaning it can be added to municipal water supplies. The deadline for submissions is this Friday.

The law change comes after a High Court justice said the law needed clarification, as he ruled against an appeal by anti-fluoride groups to stop fluoride being added to water.

David Sloan, the director of anti-fluoridation lobby group New Health New Zealand, said the Ministry of Health was rushing the law change. But the organisation's request to extend the timeframe for submissions outside of the Christmas and New Year period was dismissed by the Ministry of Health.

"We're a voluntary organisation and all have jobs, so to expect us to put together a good submission in that timeframe is terrible," Sloan said. "This is kind of a sinister move by the Ministry of Health."

The change comes after New Health New Zealand sought a judicial review against the Attorney General, arguing that fluoride was subject to the Medicines Act.

In his judgment, Justice David Collins said he would not focus on the safety of fluoride but on whether or not it could be defined as a medicine. Added to domestic water at 1.5 milligrams per litre, the concentration was so low it could not be considered a medicine. But at higher concentrations it would be a medicine - so officials should clarify the law.

Dr Loyola Correa, a dentist for 35 years, said the law change was just commonsense. Fluoride was more like a health supplement than a medicine - but it was essential for the health of teeth.

"Fluoride is the only long-term way to prevent decay across a population," Correa said. "It is essential to prevent ongoing health problems that can last throughout a person's life."

Brushing with toothpaste was not enough. "For the first two years children do not brush their teeth while their teeth are forming," he said. "Then when they do start using toothpaste it is only treating the baby teeth. So their permanent teeth won't get the beneficial effect from the toothpaste until they are six years old."

Auckland mum Nicola Orton said she wasn't concerned about her two children drinking water treated with fluoride: "I'd rather my kids drank water than Coke," she said. "It's just a glass of water which seems to be healthy."

The government will decide on the proposed amendment in early February.