Labour leader Jacinda Ardern has shown a ruthless streak on her third day as leader, effectively forcing co-leader Metiria Turei to rule herself out of a future ministerial role in a bid to stem the ongoing ructions of Turei's welfare admissions.

In a double-blow for the Greens, Ardern has also chosen the environment as one of the top four policy areas she intends to ramp up in her bid to stamp her own brand on the campaign - a signal she is targeting environmentally-conscious voters in the cities, a core voting pool for the Green Party.

After taking over as leader on Tuesday, Ardern gave herself 72 hours to review the Labour campaign. The results of that were released yesterday and Ardern said it would mean an upgrade to policies on infrastructure, the environment, education and health.

Ardern will make her first policy announcement on Sunday, expected to be on Auckland transport and include a rail link from the airport to the CBD. An environment policy will follow next Wednesday.

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Ardern said she wanted the campaign to reach people who didn't vote last election and would be using social media a lot more. Tertiary students are expected to be the main target of Ardern's first big education announcement - she has previously said she was concerned about the cost of education and cost of living for students.

However, she said she did not intend to blow the budget on big policy moves. She intended to stick to the promises Labour made in the Budget Responsibility Rules signed with the Greens, and to the limits of the fiscal package Labour has already released setting out its spending plans and forecasts for debt reduction and surpluses.

However, she would not rule out a change in Labour's previous stance not to change income tax rates until after a tax working group had reviewed them in its first term in Government. She was repeatedly asked if some New Zealanders would pay more tax under Labour and if the top tax rate would rise, but refused to comment, saying "all in good time".

National 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.

"I think every leader that comes in gets a honeymoon ... it is really what they do with that opportunity." He said every election he had been involved in had been "a big scrap".

Ardern brought both her potential coalition partners into line - saying yesterday morning that she was ruling out any chance of NZ First leader Winston Peters serving a stint as Prime Minister, and then moving on to the Greens. Ardern revealed she told her team yesterday morning to advise Turei that she could not include her in a Cabinet role.

That came after further revelations about Turei's time on welfare in the 1990s, and the admission she had enrolled at her ex-boyfriend's flat so she could vote for a friend of hers. Although Ardern denied she had forced Turei's hand, in effect that left Turei with little choice.

Yesterday afternoon, Turei held a press conference to say she would not be seeking a ministerial role, although she did not reveal Ardern had made it clear it would not happen.

Ardern insisted the bond with the Green Party remained strong and the memorandum of understanding between the two parties was still intact, describing it as "an incredibly sad set of circumstances".