New Labour leader Jacinda Ardern's rise to Labour leader happened so quickly, she did not even have time to tell her partner Clarke Gayford.

Speaking to the Herald a few hours after she was elected unopposed to replace Andrew Little as leader, Ardern, 37, still had not told Gayford. He had been away filming and she had not been able to get in touch with him.

She said she had been genuine in the past when she said she was not ambitious for the top job. That was partly because she was thinking of having children and the demands of the job were so great.

Yesterday she got that job anyway.

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Asked about those other plans now she was aiming to be Prime Minister, Ardern said there was "no doubt" it would be tough to do that and be a mother.

"But it's difficult for someone who's a partner in a law firm, someone who's a teacher, someone who's working three part-time jobs to juggle that and being a mother.

"I don't place myself in any greater vexed issue than any other mother that often juggles those things.

"I don't think those issues are confined to politics. I think those are issues women in many professions have to grapple with and I just made a decision, rightly or wrongly, to grapple with that more openly than most.

"For me, I'm just taking one day at a time and I'll see where this job takes me in terms of balancing other elements of my life as well."

Ardern said she was still surprised to find herself Leader of the Opposition. While others had sounded her out about whether she would be willing to do it if Little resigned, she maintained it was Little's decision to go. "Ultimately it was Andrew's decision first and foremost, and then they looked to me as deputy."

While it had been described as the most difficult job in politics, she had a mix of emotions. She said it was humbling to be nominated by Little and supported by the caucus.

"Ultimately excited. We've got seven weeks until an election. That's the bit everybody builds up to and now I'm in a position of being the one that leads us there."

She will take a couple of days to work out what changes she will make for the campaign.

Labour MPs seem content with Ardern and her performance in a press conference after her election. Her long-time friend Trevor Mallard said that it showed she had "the policy depth of Helen Clark and the wit of David Lange".

Told of this, Ardern laughed and said it was "very generous" of Mallard.

"I think it's probably fair to say I would never personally lift myself to the gravitas of someone like Helen or David because I've always worked under them as a volunteer in various roles. But now I'm here to carve out my own path."

She said what she had managed to hide for some time was that she was a "policy wonk" who liked to get involved in the ins and outs of policies.