A challenge to Green leader James Shaw's leadership is the start of a "big shift" with the party, former MPs say.

The party voted yesterday to reopen nominations for Shaw's position, as part of the annual process of reconfirming their two leaders.

Any potential contenders now have a week to confirm a challenge against Shaw, who has indicated he is likely to run for the co-leader role again. No one else has yet showed their hand.

Former Green MP Sue Bradford said a group of Green members had been trying to topple for Shaw for years but had struggled because he had broad support from the membership and his critics had been unable to find a realistic contender to unseat him.

"The big question now is whether there's someone from outside caucus or someone who's already an MP who's willing to actually take it on," she said.

"I think it'd be tremendous for the Greens if there was someone that was willing to pick up the challenge and really be prepared to go in different direction in terms of making the Greens a truly independent party and really picking up … the thorn of climate change and becoming a real environmental as well as social and economic party again."

Bradford said that under the current leadership Greens had become "Labour light" and the co-leaders Shaw and Marama Davidson were being used to "greenwash" Labour's environmental and housing policies. The climate change agreement Shaw had struck was "feeble", she said.

Former Green MP Sue Bradford said a group of Green members had long been trying to topple James Shaw, but had not been able to find a suitable replacement. Photo / Doug Sherring.

Even if Shaw reclaimed the leadership role, the challenge against him would shape the party's future, Bradford said.

"I think over the next week there will be … a lot of thinking and conversations among Green Party members about where they really want the party to go. Do they want to stay on this very feeble course that they're on at the moment? Or do they want to pick up the difficult challenge that the original Green Party did, and really confronting existing power structures on both environmental and social and economic issues?"

Former MP Catherine Delahunty also said that some members felt Shaw had not pushed Labour hard enough, especially on climate issues.

"The world's underwater and also on fire. And that's why some people think that James' climate leadership is not enough.

"Despite his very hard work in that space, he has not actually been listening to the disaffected in his party because he's been very focused on being a minister rather than on being a co-leader of the Greens. So you have to do both if you're in that position. And otherwise, these nasty surprises can occur."

Former Green MP Catherine Delahunty said Shaw had not pushed Labour hard enough on climate action. Photo / Michael Craig

She added: "We should not be failing on climate, we should not be having increasing emissions, we should not be an international embarrassment, right, with a Green minister. So is it worth it? And that's I think one of the questions that's being asked."

Delahunty said she would not be surprised if Shaw was re-elected easily. But he should not take delegate support of around 70 per cent as a mandate to continue on the same "softly, softly" path.

"We have never had this kind of reopened nominations vote in the time that I was in the Green Party, since 1999. This is the first time this has happened without any real momentum behind it. So that tells you something's going on. And he needs to if he wants to stay there, he needs to change."

Parts of the party opposed to Shaw's leadership managed to organise a crucial 25 per cent of delegates at the party's AGM to vote in favour of reopening nominations for his position. The vote was 32 in favour of reopening nominations and 75 in favour of keeping Shaw.

Each year the Greens hold a vote to reconfirm their co-leaders for another year. Usually, those votes are uncontested, as this vote was. However if uncontested, delegates can vote to reopen nominations - essentially a no-confidence vote in the incumbent.