Jacinda Ardern says she wasn't surprised at the Green Party's decision to run in Ohariu - a move that will make it less likely Labour's Greg O'Connor will capture the seat.

"Obviously the circumstances have changed and they have looked at their position again. So, no, that doesn't surprise me at all," Ardern told media in Auckland today.

"They are of course focused on the party vote...Greg is the only candidate that has consistently asked that people vote for him. And I have every confidence that Greg will continue to run a magnificent campaign and earn every vote."

Asked if it would hurt O'Connor's chances against National MP Brett Hudson Ardern said not if people voted strategically.

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The Greens announced last night an 11th hour decision to stand Tane Woodley as an Ohariu candidate. The party told Labour of its decision shortly beforehand.

Under the Memorandum of Understanding (MoU) with Labour the Greens had decided not to stand a candidate, to give O'Connor the best chance of unseating United Future leader Peter Dunne.

That would have hurt National by removing a likely coalition partner. However, Dunne made the shock decision to quit the contest this week after polling put him well behind O'Connor.

With Dunne now removed, the Greens are standing Woodley to campaign for the party vote.

Wellington is a strong voting base for the party and they need all the support they can muster after one recent poll put them below the 5 per cent threshold needed to return to Parliament.

That poor result came after Ardern's elevation to Labour leader, and Metiria Turei's resignation as co-leader during the fallout from her admission of historic benefit fraud. Green MPs Kennedy Graham and David Clendon had earlier withdrawn from the caucus in protest at Turei remaining co-leader.

National had told its supporters in Ohariu to party vote National, but vote for Dunne and not Hudson. After Dunne's decision to retire, it is now asking for both votes and Dunne has backed Hudson to win the seat.

Speaking in Christchurch, Prime Minister Bill English said it was clear Labour had been trying to get all the Green vote they could.

"They have been quite deliberate about that and they seem to have got most of them.
It may well help us. But what's going to help us most there is having a very good local candidate, and we have a good team there. I think people will be surprised how quickly they get a campaign up and running."

In the 2014 Ohariu party vote, National received 18,810, Labour 8771 and the Greens 5623.