New Labour leader Jacinda Ardern has ruled out a post-election agreement that would see Winston Peters serve as Prime Minister.

New Zealand First's strong performance in the polls has led to speculation Peters could serve as Prime Minister in a power-sharing arrangement with Labour, should he hold the balance of power after September 23.

However, this morning on Radio New Zealand Ardern ruled Peters out as Prime Minister at any time in a Labour-led coalition.

"I am not willing to negotiate on that particular question," she told Radio New Zealand.

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"People, when they vote for Labour and National, are often looking for the two leaders with the question in mind as to who they would like to be Prime Minister.

"So I don't want to leave questions in voters' minds as to what they will get from Labour from post-[election] negotiations.

"I want clarity and transparency for them. Leaving that on the table, in my mind, does not give voters that."

Last month Peters took to Twitter to slam speculation he wanted to be Prime Minister, although he did not explicitly rule such a possibility out.

Peters tweeted such speculation was not backed by "one shred of evidence" and was "unethical, unprofessional and dishonest".

After taking over as leader on Tuesday Ardern pledged to scrap Labour's "a fresh approach" campaign slogan, and to revise some policy, but said she wanted 72 hours to consider those moves.

She is tipped to release transport policy in Auckland on Sunday, which will possibly include a pledge to build lightrail from the CBD to the airport, and will also announce Labour's new slogan and "general direction of travel".

That will focus on core issues like housing and education.