Te Tai Tokerau children finishing primary school have the worst tooth decay in the country.
But the Far North mayor says the new compulsory fluoridation order is mystifying him and happening too fast - and he still did not know how his council would pay for it.
There are no council-fluoridated water supplies in Northland, and the Ministry of Health's statistics show it.
In 2020, an estimated 60 per cent of children had tooth decay by Year 8.
She told RNZ: "Children that aren't able to brush their teeth with fluoride toothpaste, or have poor oral health, oral hygiene, are much more likely to get decay in their teeth. So they get pain, they get difficulty concentrating and difficulty eating and they're kept awake at night.
"They have to have their teeth extracted under general anaesthetic commonly if they're young children."
Johnston, the president-elect of the New Zealand Dental Association, said paying for fluoride toothpaste wasn't always possible for households in a region with some of the most extreme deprivation in Aotearoa.
"And they can't always afford to seek dental treatment, so they get no protection of their teeth from fluoride."
But fluoridation hasn't gone down well with some local government leaders receiving the letters with written orders.
Whangārei mayor Sheryl Mai has long been opposed to compulsory fluoridation of the city's drinking water.
And on August 11, Far North district councillors received a report, about their order to fluoridate the Kaitāia and Kerikeri supplies by June 2024.
In the district, more than half of adults have had at least one tooth removed due to decay, infection or gum disease, according to New Zealand Health Survey data.
But mayor John Carter did not see the need to hurry around the health order. The council asked the government last month for an extra one to two years to prepare.
"I don't understand why the sudden rush, [it] mystifies me," he told RNZ.
"It benefits everyone, but especially children, Māori, Pacific and our most vulnerable. Unfortunately, New Zealand has high rates of preventable tooth decay, with very significant inequalities.
"The Ministry of Health [and World Health Organisation] recommends adjusting fluoride levels to between 0.7 and 1.0 ppm in drinking water as the most effective and efficient way of preventing dental decay."
The ministry said it was finalising details for the $11.3m fund.
"An official from the Ministry of Health spoke to the mayor of the Far North District Council on 27 July 2022 after the direction to fluoridate was issued.
"An official from the Ministry of Health has also made contact with Far North District Council staff to answer any questions and to advise that further information on accessing the funding will follow."