NZ First made us can the water tax - Jacinda Ardern

Prime Minister-in-waiting Jacinda Ardern has revealed Labour's highly contentious tax on water for commercial purposes was scrapped during post-election negotiations.

The tax, which was widely maligned by farmers, horticulturalists and the National Party, was the only concession Labour made in its talks with New Zealand First, Ms Ardern says.

The rest, she told The AM Show on Wednesday morning, remained in place - and Ms Ardern fiercely rejected any suggestion she rolled over in negotiations and let Winston Peters get what he wanted.

"The agreement released yesterday is simply the things New Zealand First, as part of their agreement, wanted to place emphasis on," Ms Ardern said.

"Our policies, the ones we campaigned on, unless otherwise stated remain exactly the same."

However Ms Ardern admitted there was at least one concession Labour made in negotiations with New Zealand First.

"You picked last week, Duncan, one in particular - and that was the levy on water use by commercial users," she said.

"There was certainly a view by New Zealand First that that needed to be applied to those who exported our pristine water in the form of bottled water, but they disagreed on a resource rental on water beyond that.

"Our office will be investigating how to apply a royalty specifically to water bottlers."

Ms Ardern asserted strongly she would "completely disagree" with those who say she has rolled over and let Winston Peters get more than his fair share of concessions.

She says despite that one loss on water tax in talks with Winston Peters, the platform Labour campaigned on and its 100-day plan remains almost the same.

"The things that we conceded in these agreements are things we fundamentally, by-and-large, absolutely agreed with, and wanted to place extra emphasis on," Ms Ardern said.

"If you ask me, New Zealanders have conceded too much over the last nine years - these agreements are about us making sure we move forward in building a better New Zealand."

One of the core things Labour and New Zealand First did agree on, Ms Ardern says, is the establishment of a ban on foreign buyers purchasing existing houses.

"Our view is New Zealanders should be able to access their housing market and at the moment, demand is definitely problematic - as is supply," she said.

"We're in the very process right now of how we can bring that into law, what the best mechanism is, and how we can make that a reality where it affects our trade relations."

Ms Ardern also touched on the outworking of the new Government's regional development plan and its $1 billion-per-annum tree-planting policy in her AM Show interview, which she says can be paid for using unallocated capital expenditure.