National's mental health spokesman Matt Doocey says it's disappointing the Labour and the Greens have declined to work across Parliament on mental health before a major inquiry reports back.

Doocey wrote to Health Minister David Clark and all other MPs in July, calling for cross-party collaboration on what he called "one of the biggest challenges of our generation".

Doocey said today it was clear from a letter Clark sent him that he had ruled that out.

"He's actually ruled it out because he's made it very clear that he believes cross-party work is best done in a select committee. The minister is either being disingenuous or has misunderstood the role of a cross-party group," Doocey said.

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"We can find common ground and that's what I believe the public is expecting of us on a big issues like mental health, and it's disappointing that we have a health minister who's not interested in looking into that," he said.

In his letter to Doocey, Clark said cross-party work on mental health was best done during the select committee process, and after the Inquiry into Mental Health and Addiction, led by Professor Ron Paterson, had reported back.

The inquiry is due to report back by the end of October.

Doocey, who is a member of the heath select committee, also wrote to Clark on September 11, requesting a briefing on the inquiry when it was available.

Clark said he was happy to provide Doocey, and the health select committee, a briefing on the inquiry's findings "at the earliest opportunity".

Green Party mental health spokeswoman Chloe Swarbrick has also declined to consider any cross-party collaboration on mental health until after the inquiry had reported back.