Labour leader Jacinda Ardern says it's time New Zealand started having the conversation about New Zealand becoming a republic.

"I do think that we should start having the conversation. There are a lot of issues that need to be resolved on that path, and I would have liked the government to have had that conversation when the flag debate came up," Ardern said during a special PM Job Interview with the Herald today.

"That was the time to say, 'actually, where are we heading? What's the Crown's ongoing relationship with Maori if we transition into a republic? Where will we be in 20 years' time in this regard.'"

Then Prime Minister John Key said last year dismissed the chances of New Zealand becoming a republic any time soon.

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"I would be amazed if New Zealand becomes a republic in my lifetime. And I'm hoping to live a long and happy life," Key said at the time.

And this year, Prime Minister Bill English said the flag referendum showed constitutional changes such as the flag or a move toward a republic should not be led by politicians.

Ardern's interview for the Prime Minister's job came only hours after Bill English was in the same hot seat.

She fielded questions from a panel hosted by Heather du Plessis-Allan and featuring Audrey Young, Liam Dann and Toby Manhire.

Ardern also said it was her own "captain's call" to not rule out introducing a capital gains tax on rental properties or second homes next term.

Labour will wait to hear what a planned tax working group recommends on the matter, Ardern said - but waiting to act until after the 2020 election was too long. It would not apply to the family home, she said.

"It is different leadership, different decisions," Ardern said. "Andrew [Little] made a call that he would go back to the electorate.

"I made a call that if I was in government and presented with a tax working group paper that suggested these are the things you need to do to be able to tackle the housing crisis and inequality in your tax system, to then sit on that for one, maybe two years without doing anything didn't feel right to me."

She had talked to finance spokesman Grant Robertson, but it was a "captain's call".

Asked if she could rule out raising top tax rates if in government, Ardern said she wanted to wait until Treasury tomorrow issued its Pre-election Economic and Fiscal Update before making such decisions.

On trade policy, Ardern would not rule out pulling out of the TPP if the 10 other countries refused to renegotiate if Labour was in power after the election.

The main concern for Labour was that the TPP did not allow for Labour's policy of banning foreigners from buying New Zealand land. One of the key dates for the TPP11 talks will be APEC in November when talks will take place on the sidelines.

Asked whether Labour would pull New Zealand out if the other countries did not agree to renegotiating at that point, Ardern said Labour's view was New Zealand should be able to renegotiate.

"To date we haven't seen any effort to put that on the table. Australia has negotiated these exemptions. Why can't we? The idea this is impossible, I do not buy."

Ardern said Labour had always been a party that championed trade, and saw it as critical to New Zealand.

The Labour leader was asked by Herald political editor Audrey Young if she was being bloody-minded, given it would be possible to effectively ban foreigners from buying property through a stamp duty.

"I'm happy to be accused of being bloody-minded when it comes to improving our export and trade conditions. Also bloody-minded when it comes to protecting New Zealanders' ability to get into a home. I don't think they are mutually exclusive. I'm broadly bloody-minded."

Ardern has committed to immigration policy announced under previous leader Andrew Little that the party estimated would cut net migration numbers by about 20,000 to 30,000 a year. Today, she said she didn't like to fixate on numbers, and made an explicit promise to businesses.

"My message to businesses is if you have a genuine skill shortage, you will be able to access migrant labour to fill it."