"Let's do this." - Labour has unveiled its new campaign slogan, retiring "a fresh approach" along with Andrew Little's leadership.

New leader Jacinda Ardern said the new tag-line had come about almost inadvertently, the sign-off she wrote in a social media post shortly after becoming leader.

"It's not just about the next seven weeks. It is about what we can do if we are in a position to make change. I'm excited about this campaign, and what is possible - so, let's do this."

On market research behind the new slogan, Ardern said, "ah, we have asked ourselves", and a few ideas were "thrown around".

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"We noticed we had actually started using this tag-line already."

The slogan will be featured on new party billboards and campaign advertising along with a photo of one person only - Ardern.

At a press conference this afternoon new deputy leader Kelvin Davis joked he was just fine with that.

"I've got the easiest job in politics. I just need to kick back and bask in the glow of the Jacinda effect."

Ardern has promised to change some policy since taking over as leader. The first policy announcement under her leadership will be transport-related on Sunday - thought to include a pledge to build light rail from Auckland CBD to the airport.

Another environment-related policy will be unveiled next Wednesday.

She would not reveal other policy changes. The new Labour leader was repeatedly asked if some New Zealanders would pay more tax under Labour and if the top tax rate would rise, and said she would not comment: "all in good time".

Ardern said she wanted the campaign to reach people who didn't vote last election - a population the size of Hamilton.

"That has to change. We will be using different tools, every tool available. I will for instance be hosting Facebook lives every single week."

Mike Munro, who worked for the Helen Clark government and Wellington Mayor Justin Lester, has been brought in as strategic advisor.

In March, Labour and the Greens announced an agreed set of economic rules - the "Budget Responsibility Rules", which include a commitment to run surpluses every year. Ardern said Labour would be sticking with those rules.

National's campaign chair Steven Joyce said Labour had clearly been struggling, and he would expect a poll bounce after Ardern's elevation.

"I would certainly expect them to pick up a bit of vote and a lot of that will come back from the Greens and New Zealand First. But as to the extent of that, that is yet to be determined."

Asked if the Jacinda effect was real, Joyce said every leader brought something but it was more about the people of New Zealand and what was important to them.

"I think every leader that comes in gets a honeymoon...it is really what they do with that opportunity.

"Prior to this change it was just too calm and too quiet. But every election campaign I've been involved in, and this is my fifth, they are always a big scrap."

The Jacinda effect - even Creature is a fan

Labour's deputy leader Kelvin Davis was asked at today's press conference about the reaction to Ardern becoming leader.

The Te Tai Tokerau MP, who has recently attended a tangi, said his new boss had even secured the enthusiastic backing of his cousin.

"His nickname is Creature. His real name is Jimmy. We were grave-digging and he goes, 'Kelvin, I'm voting for Jacinda' ... if you can get Creature interested in voting ... she has nailed it."

Asked how his cousin got his nickname, Davis said that was a long story, "but if you meet him you will know why we call him that."