Prime Minister Jacinda Ardern has refused to rule out introducing a new wealth tax as Labour overhauls its tax policy – leading to accusations she might backtrack on a pledge in 2020 not to introduce one.

On Newshub's AM Show today, Ardern reiterated her pledge not to introduce a capital gains tax while she was Prime Minister but did not rule out a wealth tax, saying Labour was yet to form its 2023 tax policy.

In the 2020 election campaign, Ardern was asked if she would resign as Prime Minister if her government did implement a wealth tax and replied: "I won't allow it to happen as PM."

That had come after questioning about the Green Party's policy for a new 1 per cent tax on those with a net wealth of more than $1million.

Ardern repeatedly ruled that policy out and when asked the wider question about whether she would resign if she introduced a wealth tax, she said it was hypothetical because it would not happen.

"I have made my position and the Labour Party position absolutely clear. We have ruled it out. This is not up for discussion. It's not in play. There is no need for the hypothetical. It won't happen."

Pushed again on the same question, she answered: "I won't allow it to happen while I am Prime Minister."

She said no other countries had that form of taxation. "Now is not the time to be experimenting with tax policy when we need to focus on our economic recovery."

However, she appeared to have forgotten that when asked about that pledge on the AM Show on Monday.

While she repeated the capital gains tax promise, on the wealth tax she said Labour had not yet formulated its policy for the 2023 election and was looking at where the tax burden in New Zealand fell.

Her new comments come as Revenue Minister David Parker probes into the taxes that the wealthiest in New Zealand are paying. He has also proposed a new Tax Principles Act to set out principles to assess tax policies under – a mechanism to ensure governments assessed the fairness of policies.

Ardern said she did believe aspects of the tax system were unfair, and Parker's work was looking at that. She repeated her promise that Labour would not introduce tax changes this term and would campaign on them in 2023 once it had set its policy.

National's finance spokeswoman Nicola Willis said Ardern seemed to be signalling a broken promise was coming.

"Despite telling New Zealanders during the 2020 election that she would not implement a wealth tax as long as she is Prime Minister, Jacinda Ardern is now signalling a U-turn on that promise.

"Labour is addicted to spending and is now looking for any excuse to load more taxes on Kiwi households and businesses to pay for it.

"The writing is on the wall. David Parker is collecting wealth data and is talking about wealth taxes."

In 2020, Ardern had criticised the National Party for continuing to claim that a Labour-Green government could introduce the wealth tax, saying it was "desperation tactics and misinformation".

Labour minister Stuart Nash at one point also said he would resign if the wealth tax came in.

Ardern had already ruled out a capital gains tax after NZ First blocked efforts to get it over the line, but had said she continued to think it was a good idea.

She said the wealth tax was very different.