Prime Minister Jacinda Ardern has ruled out lowering the MMP threshold in time for next year's election.

Photo: RNZ / Rebekah Parsons-King

National leader Simon Bridges this morning challenged Ms Ardern to take the option off the table following the Green Party's push to reduce the limit from 5 percent to four percent.

On her way into Parliament this afternoon, Ms Ardern took the opportunity to do so.

"Anything that we may do in this space, no, would not happen for the 2020 election.

"Obviously this is about the future makeup of parliaments and having it determined in that way I don't think would be appropriate."

Green Party co-leader James Shaw said New Zealand should adopt the full recommendations of the Electoral Commission's 2012 MMP Review, but agreed that should wait until after the 2020 vote.

"That deals with any public perception that you're looking after your own interest."

Mr Shaw said the proposed changes were relatively minor and the Electoral Commission had previous said they did not need to go to a referendum.

He said, however, he'd be open to a public vote if necessary.

"Ultimately, if people feel that it needs to go to a referendum in order to have that legitimacy, then I'd be okay with that too."

New Zealand First leader Winston Peters said his party supported the current 5 percent threshold, but was happy to ask the public's view.

"We'll put it to the New Zealand people, not decide it ourselves," Mr Peters said.

Earlier in the day, Justice Minister Andrew Little declined to go as far as Ms Ardern, only saying that any change before next year's election would be "unlikely".

He said the governing parties were still in discussion, but he believed there was "an acceptance that we should change those thresholds to make MMP fairer".

Mr Little said he was sympathetic to the idea of putting the matter to a public vote alongside questions about legalising cannabis and euthanasia.

"The voters having a say takes away any allegation that this is politicians acting in their self-interest."

Any changes before the next election would be 'rorting the system' - National

National Party justice spokesperson Mark Mitchell said any change should first have to go to a referendum.

Photo: RNZ / Rebekah Parsons-King

"I don't think there should be any change like that made unless the people of New Zealand actually support it."

Mr Bridges said he was "happy to talk" about electoral reform with the Prime Minister, but any changes before the next election would be "rorting the system".

He said most of his caucus would support the threshold remaining at 5 percent and described the move as a "stalking-horse" designed to "save the three-party coalition's bacon".

At the moment, political parties need to win the support of at least five percent of voters or win an individual electorate seat to make it into Parliament.

The most recent 1 News Colmar Brunton poll put the Green Party on 6 percent support and New Zealand First on 3 percent.