Jacinda Ardern is "absolutely" confident that the new coalition government will last the full term.

The new Prime Minister will hold her first caucus meeting this afternoon, when ministerial roles will be discussed. Labour has agreed to give New Zealand First four Cabinet positions and an undersecretary, while the Greens will be given three ministers outside Cabinet and an undersecretary.

At just the age of 37, Ms Ardern is the country's youngest prime minister in 160 years, New Zealand's third female prime minister and joins the growing ranks of young leaders around the world.

Even her opponent Bill English yesterday called her rise to the prime ministership "remarkable", after she took over the Labour leadership just seven weeks out from Election Day, leading the ailing party to an immediate surge in the polls.

She told RNZ's Morning Report the coalition would "absolutely" last the distance.

"There's a couple of reasons I can say that with confidence. The first is the nature of the agreements. The second is the relationships I've formed with the leaders of both parties - one [with James Shaw] is long-standing and the other has a very firm foundation based on these negotiations."

The final reason was that Labour and New Zealand First had been in government together successfully before, Ms Ardern said.

"Mr Peters and New Zealand First were a party of their word. They provided stability and we presented and delivered a stable government in 2005 ... [that] went the full distance."

It was "unfair" to characterise the new government as a coalition of losers.

"This is an MMP environment. We've formed a coalition government based on the majority of votes and based therefore on what the majority of New Zealanders sought in this election."

Photo: RNZ / Rebekah Parsons-King

Labour backs away from water tax

Details of support agreements with both New Zealand First and the Greens would be released early next week, Ms Ardern said.

The agreement with the Greens was "essentially complete" but a few details in the agreement with New Zealand First needed to be tidied up.

However, Labour's plan for its first 100 days in government had remained mostly intact, "with a few minor changes", she said.

She would not confirm ahead of releasing the agreements whether a water tax remained on the table, but appeared to be backing away from the possibility.

Mr Peters said during the campaign that he would not support a water tax for farmers and other agricultural users, but would support a royalty charge on water bottling.

"We remain committed to cleaning up New Zealand's rivers but I absolutely reflect now that Mr Peters did take a strong view on the mechanism that we were choosing to use," Ms Ardern said.

"So you will see what has happened in that final agreement ... but that was the firm view he took."

Despite not releasing the agreements immediately, there would be "plenty" of other announcements this week, she said.

"We do need to release the ministerial portfolios that will be held by our support partners and then we'll be releasing the fuller ministerial portfolios later in the week."

The new Cabinet would have 20 ministers, with a further eight outside Cabinet.

It was Winston Peters' decision as to whether he took up the role of deputy prime minister, she said.

"I wouldn't have offered it to him if I did not think that was going to be something that would provide stable, workable, solid government."

She guessed that Mr Peters would choose Labour during his announcement preamble, when he spoke about wanting all New Zealanders to share in the country's economic prosperity.

"That was the moment I thought, 'he's going with us.'"

And her first meal as Prime Minister of New Zealand? "I headed straight back to my studio apartment in Wellington and had a pot of noodles."