The Prime Minister has been forced to clarify Labour's stance on fluoridation, after it was confirmed two Government MPs plan to host an anti-fluoride group at Parliament.

Prominent US anti-fluoride campaigner Paul Connett is understood to be briefing MPs at Parliament in the coming weeks, hosted by Labour backbencher Duncan Webb and NZ First MP Jenny Marcroft.

Legislation to allow DHBs, rather than district councils, to decide whether a town water supply should be fluoridated, is before the House awaiting a second reading.

SUPPLIED Fluoridation has been labelled one of the greatest health advances of the 20th century.

It was introduced by the last Government and passed through an extensive public select committee process, before the committee MPs recommend it be passed with some minor additional funding changes.

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Public drinking water currently supplies about 85 per cent of the population. Of those on public water supplies, 54 per cent receive fluoridated water. New Zealand had high levels of tooth decay and placing the issue under the authority of health boards was expected to increase fluoridation across the country.

MAARTEN HOLL/STUFF National health spokesman Jonathan Coleman has questioned why two Government MPs are hosting a group that peddles "junk science".

Connett is to be brought to New Zealand by Fluoride Free New Zealand, to hold a number public briefings on their position throughout the regions.

But National Party health spokesman Jonathan Coleman has said the information being peddled by the group and Connett is "junk science" and dangerous.

He questioned why those views would be treated as expert opinion in a briefing to MPs.

KEVIN STENT/STUFF Prime Minister Jacinda Ardern says her party has not changed its stance on fluoridation, despite two Government backbench MPs hosting an anti-fluoride movement at Parliament.

"I'm very disappointed to see that that's happening, and the question to the Labour Party has to be do they support fluoridation or not?

"At the time the Labour Party broadly said they supported that. Now you've got two Government MPs hosting anti-fluoride campaigners in Parliament... and the fact that we've got two Government MPs hosting that meeting would seem to indicate they don't any more," he said.

Fluoridation has been labelled one of the greatest health advances of the 20th century by the World Health Organisation. Coleman said the issue was "clear cut".

"They need to say if they support fluoridation or not. If they support fluoridation, which is the single best way to improve the oral health of children and adults in New Zealand - but especially adults - they wouldn't be listening to the junk science and the dangerous views that Fluoride Free New Zealand are promulgating.

"A lot of the stuff they're saying is extremely misleading. But the weight of medical and scientific experience supports fluoridation so I don't know why these MPs are spending their time there."

Prime Minister Jacinda Ardern said the meeting was discussed by the Labour caucus before it was agreed the MPs could host the group. But that was in the interests of a representative Parliament.

"We had a caucus discussion. Someone made an approach, they wanted to be able to offer a briefing to MPs - many groups ask for the opportunity to brief MPs on issues.

"And they'll be issues we don't always agree with, it's just whether or not we allow them access to talk openly about their position on an issue," Ardern said.

Labour had not changed its position on fluoride.

"It does not change our view and Health Select Committee has also provided access to those groups to share their view. It does not mean we've changed our position. It's just about letting people have their say regardless of whether we agree with them or not."