The mayors of two Wairarapa districts facing water fluoridation as default both said they would prefer consumer choice on the issue.
Last week, the Government announced plans to amend the Fluoridation Bill, first introduced in 2016.
Associate Health Minister Dr Ayesha Verrall said adding the chemical to drinking water would boost the nation's oral health.
Verrall, who visited Wairarapa last weekend in her other role as Minister for Seniors, said the amendments would put the decision in the hands of the director general of health's office rather than local councils or district health boards.
She said the Fluoridation Bill "recognises water fluoridation is a health-related issue".
"Community water fluoridation is a proven public health measure that will make a big difference to children's wellbeing."
Verrall said local councils would be responsible for fluoridation's capital and operational costs, but funds would be available to support related infrastructure work.
However, Carterton Mayor Greg Lang and South Wairarapa counterpart Alex Beijen both said they would prefer councils and ratepayers to make that decision.
Beijen said he was happy with the status quo, "where local people could choose additives to their water".
"However, as central government continue to move control of local government matters to government departments, this oversight is removed from a forum where public have their say."
Lang said his personal view "is that fluoride should be a consumer's choice".
"But I will accept the decision of the director general if his decision is to add it to our water supply.
"I believe the director general of health [is] best placed to make the call based on scientific evidence and health benefits.
"In terms of impacts on Carterton residents, depending on [the] decision, people can either access [their] own fluoride in the form of tablets, etc., or it will be added to water."
Masterton's councils, first borough then district, have added the chemical to its supply for almost 50 years.
The urban supply makes up 45 per cent of the population of the Wairarapa District Health Board [DHB] region.
In its long-standing position statement, the DHB said it endorses community water fluoridation [CWF] "as an effective public health measure contributing to oral health maintenance, prevention of tooth decay and reduction in health inequalities".
"It is important that local authorities continue to receive evidence-based advice from health professionals at their local DHB, who are committed to improving oral health and the overall health status of their communities."
The DHB statement said "small but active" local groups remained against CWF.
In its statement, published on Tuesday [March 23], the Fluoride Free NZ organisation said people should "be stunned" about the move.
The group's Paul Connett and Vyvyan Howard said that other risks trump oral health benefits.
"Why on earth is NZ not putting an immediate halt to existing fluoridation programmes instead of making it mandatory for the whole country?
"And why are NZ's scientists and educators leaving it to voluntary organisations like Fluoride Free NZ to issue critical warnings?
"A decayed tooth can be fixed, a damaged brain cannot."
However, others said the move was overdue.
Professor Barry Borman, director of Environmental Health Indicators NZ at Massey University, said, "it is about time".
"This should have been done years ago.
"Part of the issue is there's been a lack of national leadership in this. It's like a lot of things in health - the issue's far too important for there not to be a national approach.
"Anti-fluoridation advocates are going to be opposed to this. But nobody has really put up epidemiologically-sound evidence to support their views. The health outcomes are the thing we need to be addressing in a consistent manner."