Mental Health Foundation chief Shaun Robinson said he had concerns, given there was already a "common, unsubstantiated belief" gun violence was linked to mental distress.
"In fact the evidence is clear that people experiencing mental health issues are far less likely to be involved in perpetrating violence. Rather, they're more likely to be the victims of violent crime," he said.
"The proposed legal provision is likely to perpetuate damaging stigma and discrimination for those living with mental distress."
Robinson said he was also concerned the provision could lead to people not discussing their mental health with GPs for fear of being seen as unfit.
"Anything that creates a barrier to people seeking support is highly counterproductive," he said.
But the New Zealand Medical Association says doctors already have similar rights and obligations for other potential risks.
"One of the other situations where mandating of personal information happens is where they have concern about accidental injury to children. Does that mean parents won't bring their children into doctors? That hasn't occurred," NZMA chair Kate Baddock said.
She said the suggestion people would not go to a doctor because their mental illness would preclude them from having a weapon was "stretching the information a little further than credibility".
Doctors were already allowed to pass on information in the event of child abuse, risk of suicide and risk of harming others, Baddock said.
"Where that applies we have a mandated responsibility to inform. So there's quite a wide range of times when you need to consider whether it's the right thing to do."
"It's not anything new for doctors."
Police Minister Stuart Nash has declined to comment on the details of the leaked document but says a final version has yet to be signed.
"If there are any gaps I expect the Select Committee to identify and work through these issues," he said in an earlier statement.
The major components of the upcoming reforms will be an individual gun register, significantly stricter compliance penalties and new rules for gun clubs.
The National Party supported the first tranche – which banned military-style, semi-automatic rifles – but has caused a stir by saying it is likely to vote against the new reforms.
Opposition leader Simon Bridges this week told reporters, based on the leaked draft, the bill would penalise law-abiding gun-owners while not being tough enough on criminals.
The Police Association has accused National of playing politics, something Bridges denies.
The bill has yet to go before Cabinet.
