The Green Party has cast doubt on suggestions it can be relied on to support National's bill to create the Kermadec Ocean Sanctuary.
National MP Nick Smith has lodged a member's bill banning all mining and fishing over 620,000 sq/km around the Kermadec Islands.
Dr Smith said he was confident the proposed law would have the support to pass, with National's 56 votes and the Green Party's eight.
"The Greens have indicated to me their support for any bill that would put the sanctuary in place," he said.
But a Green Party spokesperson said its MPs had yet to consider how they would vote on Dr Smith's bill.
The spokesperson said the party's priority was for the government to progress the scheme, as noted in its confidence and supply deal with Labour.
That agreement included a commitment to "use best endeavours and work alongside Māori" to establish the Kermadec Ocean Sanctuary.
That could prove difficult though.
New Zealand First has previously objected to the plan, saying it curtailed Māori fishing rights.
And the government has put any legislation on hold until a resolution can be found that is satisfactory to all parties.
Dr Smith said he was worried Labour would end up putting the sanctuary in the "too-hard basket".
He denied he was stirring up trouble and trying to drive a wedge between the government support parties.
"It's got to be bigger than party political games ... this is National continuing to want to do the right thing for improving protection of New Zealand's ocean area."
The proposed Kermadec Ocean Sanctuary has been in troubled waters since former Prime Minister John Key announced plans for the marine reserve in 2015.
The legislation was delayed after widespread opposition from Māori and the fishing sector.
In 2016, the then-National government put the legislation on ice until this year's election after the Māori Party withdrew its support.
The sanctuary would be one of the world's largest fully protected areas, covering 15 percent of New Zealand's ocean environment.