The Labour Party's pact with the Greens is to blame for its demise, NZ First leader Winston Peters says.

"When the Labour Party signed the [Memorandum of Understanding] I knew it was all over rover for them," he said at Parliament today.

Peters said it was not former Labour leader Andrew Little's fault that the party had dropped in the polls because it was inevitable that the deal with the Greens would hurt his party.

"If you begin from a position of weakness, that's all you'll be, is weak."

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He made the comments after Little resigned this morning, and was replaced by deputy leader Jacinda Ardern.

Little stood down after a series of polls which showed Labour was falling in support, partly by losing votes to the Greens.

But Greens co-leader Metiria Turei rejected the suggestion that her party had cannibalised Labour's support after signing the MOU.

She also said the agreement was unaffected by Labour's change of leadership.

Little was a man of "great integrity", she said, and she was sad that he was no longer leader.

"But I've also worked with Jacinda Ardern over a number of years in Parliament. I hold her in very high regard."

The strongest critic of Ardern was Act leader David Seymour, who said she had done nothing in her nine years in Parliament apart from asking questions in the House.

It was not credible to have a 37-year-old Prime Minister in coalition with Turei and Peters, he said.

National MPs said Labour's change of leadership would not alter their approach to the election.

"We always respect the Leader of the Opposition," Prime Minister Bill English said.

"This is someone who in a matter of months could be Prime Minister. And in the context of the current campaign we need to lift our levels of support."

"This doesn't change anything," National's campaign chairman Steven Joyce said.

"There's stacks to do between now and the election. I've done a few of them, I know how hard you have to work."

Ardern's former rival in Auckland Central, National MP Nikki Kaye, went on the attack when Ardern became deputy leader, saying the party had made a "cosmetic facelift".

She was more gracious today, saying that she had texted her congratulations to the new Labour leader and "wished her all the best".

Ardern has also had a rivalry with deputy Prime Minister Paula Bennett, as the two previously clashed in their welfare roles.

Bennett made a sly dig today at Ardern's past reluctance to lead the party or the country.

"She's always said she doesn't want that job and yet she's found herself there today."

Asked whether she was jealous that Ardern had beaten her to party leader, she said: "No, I'm quite all right with that."