The first of up to 90,000 Northlanders likely to soon be drinking water dosed with industrially-produced fluoride could be doing so within 18 months.

In July last year, former director general of health Dr Ashley Bloomfield ordered government-mandated fluoridation for Whangārei and Far North council drinking water plants servicing all but one of the region's biggest population centres - in Whangārei, Bream Bay, Kaitāia and Kerikeri and affecting 65,250 people.

The government then in November indicated further mandated fluoridation was in its sights for more Far North drinking water supplies including those for Kaikohe, Kawakawa, Moerewa, Ōkaihau, Ōpononi, Paihia and Rawene - and potentially affecting another 14,180 people.

It has also asked for high-level fluoridation cost estimates for two Kaipara council drinking water supplies, but has not yet mandated the chemical's addition to any of its supplies. Adding fluoridation to Dargaville and Maungaturoto drinking water would add a further 7600 affected people into the mix for the North.

None of Northland's 17 council drinking water supplies is currently fluoridated.

The government says Whangārei District Council (WDC) and Far North District Council (FNDC) have until July 2024 to start putting fluoride into the seven water treatment plants that were part of the July 2021 government order.

If fluoridation goes ahead, councils will be using New Zealand-made industrial fluoride in the form of fluosilicic acid, also known as hydrofluosilicic acid.

Hydrofluosilicic acid is produced during the manufacture of superphosphate fertiliser.

It is the form of fluoride most widely used by New Zealand councils, added into drinking water as a liquid.

WDC and FNDC have told the government it will cost more than $5 million to fluoridate their July-mandated water treatment plants - WDC $3.75m and FNDC $1.32m.

They are among 14 councils that were mandated to fluoridate in July which will be competing for the government's $11.3m Manatū Hauora Ministry of Health fund. This was set up for councils for drinking water fluoridation introduction costs.

The two Northland councils' costs alone amount to more than 40 percent for the available money.

WDC water services manager Andrew Venmore said his council was waiting to hear back from Manatū Hauora about funding.

WDC's fluoridation start date would depend on whether Manatū Hauora Ministry of Health funding was provided to the council, he said.

WDC's mandated five water treatment plants affect more than 65,250 people.

In Bream Bay, 10,250 people would be impacted via the council's plants at Ruakākā and Ahuroa in Waipū.

The three plants classified as Whangārei and affecting 55,000 people are the Whau Valley plant, Ruddells in Cemetery Road Maunu and Poroti.

Venmore said once funding was secured, it would be as soon as 18 months for fluoridation to kick in for the council's Whau Valley and Ruddells water treatment plants, 24 months for the Ahuroa and Ruakākā water treatment plants and 42 months for the council's Poroti treatment plant.

The council would have to "seek funding from within our own resources" if government funding was not available or enough for the job to be done, Venmore said.

That could take six months.

WDC would also have to spend $100,000 annually to run its fluoridated drinking water supplies, he said.

Venmore said WDC would have a fluoridation dose rate target of 0.85mg/l.

Meanwhile, FNDC's July-mandated water treatment plants' fluoridation affecting 12,100 people would be in Kerikeri 6700 and Kaitāia 5400.

Additionally mandating fluoridation the FNDC plants signalled by the government in November would affect 14,180 people - Kaikohe 4200 people affected, Paihia 4000 people, Kawakawa-Moerewa 3500 and Rawene-Ōpononi-Ōkaihau 2480.

FNDC manager infrastructure operations Glenn Rainham said fluoridating the Kerikeri and Kaitāia plants would add another $42,000 annual council operating cost on top of the initial capital outlay.

There would be more costs on top of that if other treatment plants were mandated.

"If a new direction order is given, it is the council's understanding that the cost will be met by new water entities planned by the government," Rainham said.

The government ordered compulsory fluoridation for 14 councils in July 2021 and four months later indicated to 27 councils it was considering the option for more of their drinking water supplies.

Meanwhile, KDC operations manager infrastructure services Donnick Mugutso said his council had not been mandated to add fluoride to its Dargaville and Maungaturoto drinking water supplies.

He said KDC had done initial high-level fluoridation implementation cost estimates as requested by the Ministry of Health. It would do more detailed costings if directed to fluoridate.

KDC has told the government it would take about three years to bring in fluoridation - if a direction was given and funding was available.

The council would seek Manatū Hauora funding if told to fluoridate.

"We would seek as much as could be allocated, to relieve any potential cost impact fluoridating water supplies would have on our ratepayers," Mugutso said.

Mandated fluoridation for KDC water treatment plants would affect more than 11,000 people - Dargaville 6800 people and Maungaturoto 1100 people.

More than 70 percent of Kaipara is on private water supply.

The Ministry of Health notes fluoride has been widely used to top up fluoride to optimal levels for more than 60 years.

"While there is a large body of evidence of its significant benefits to oral health, there is no evidence linking optimal fluoridation with any adverse health effects," it said.

Hydrofluosilicic acid is produced while manufacturing phosphate fertilisers. Phosphate rock, which contains fluoride and silica, is treated with sulphuric acid. This produces two gases: silicon tetrafluoride and hydrogen fluoride. These gases are passed through scrubbers where they react with water to form hydrofluosilicic acid.