Opposition health spokesman Jonathan Coleman says it's disappointing to see Government MPs hosting promoters of "junk science" at Parliament.
Labour's Duncan Webb and New Zealand First's Jenny Marcroft are hosting an "expert briefing" from Fluoride Free New Zealand this week.
Health Minister David Clark said he will not be attending the briefing, but it was OK for others to go along.
"MPs hear from a wide range of people, it's part of our job to make sure we are listening to all different sides of debates.
"I think one meeting with a group will not undermine science, no."
Prime Minister Jacinda Ardern said many groups asked to brief MPs on issues and hosting them did not mean the Government agreed with their stance.
"It's just about letting people have their say regardless of whether we agree with them or not."
But National Party health spokesman Dr Coleman said that wasn't good enough.
"Parliament has to be a place where there's an exchange of views, but these are hardcore campaigners that misrepresent the science, and that's the problem.
"This is now going to unfairly colour the debate and raise questions in people's minds, especially MPs who are going to vote on these Bills."
He said it was disappointing to see Government MPs hosting Fluoride Free NZ.
"If the Government supported fluoridation, they wouldn't be listening to the junk science and the dangerous views that Fluoride Free New Zealand are promulgating.
"I suspect it's a case of two new MPs rather naively agreeing to host this briefing and now going to be caught up in what's going to be quite a controversy."
National MP Andrew Falloon tweeted a copy of the meeting invitation, asking "Hey @Duncan_Webb_ does this mean you're anti-fluoride?"
Mr Webb defended himself, tweeting that the whips of each party had agreed it was appropriate to give the group an opportunity to speak at Parliament, and that he would "do the same for either side of the debate".
A 2014 report commissioned by the Prime Minister's Chief Science Advisor Sir Peter Gluckman and the Royal Society of New Zealand found fluoridation of water at the established levels in New Zealand had broad benefits for dental health.
It also concluded there were no adverse effects on health as a result of fluoridating public water supplies.