Prime Minister Jacinda Ardern has slammed the National Party over its stance on the UN migration compact, but is stopping short of calling for Simon Bridges to resign over it.

And while she wouldn't say whether she thought National was trying to court far-right voters, she did not dispute Winston Peters' description of the movement opposing the compact as neo-Nazi.

Peters, leader of New Zealand First and Deputy Prime Minister, called for Bridges' resignation at the weekend after it was revealed that police were investigating death threats made against him.

This followed video footage, revealed by Newshub, from the start of the year of a Christchurch protest against the compact in which protesters called for Peters to be hanged.

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Among the protesters was Philip Arps, who has been sentenced to 21 months' jail for sharing the gunman's video of the Christchurch terror attack. He also delivered a pig's head to Al Noor mosque in 2016.

The Government signed up to the compact at the end of last year despite claims from the National Party that it would restrict the ability of future governments to decide on which migrants were welcome and which weren't.

Legal advice from Crown Law and the Ministry of Foreign Affairs and Trade confirmed that the compact was not legally binding, nor did it restrict New Zealand from setting its own migration policies.

But National leader Simon Bridges stood by his earlier stance, saying in a statement that New Zealand should decide its immigration settings, not the United Nations.

He noted that the US and Australia have not signed the compact.

"This Government has become more concerned with impressing the UN, rather than what matter to New Zealanders - and that's immigration being decided by us."

Peters told Newshub that the misinformation around the migration pact, particularly the incorrect view that it would be binding on signatories, was based on propaganda from Austrian neo-Nazis, in particular far-right movement leader Martin Sellner.

That made Bridges' position as leader untenable and he should resign, Peters said.

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Ardern would not say whether Bridges should resign when asked by media today, but she said National's position on the compact was irresponsible.

"I did hold concerns because we weren't having a debate that was anchored in the facts.

"They claimed that we wouldn't hold sovereignty over our own borders - that was totally factually incorrect and remains incorrect. We would never sign away the sovereignty of the maintenance of our own immigration policy.

"What they were saying was just wrong. And the movement around it was by a very particular group who had a very particular desire, and you can see by those who jumped on that bandwagon what they were trying to do."

Asked whether National was trying to appeal to the far-right voters, she said: "That's really ultimately for them to answer."

She said she hadn't seen that kind of position from "mainstream political parties in New Zealand".

Former NZ First MP Tau Henare questioned on Twitter whether it was ironic for Peters to call for Bridges to resign.

The National Party set up an online petition opposing the compact, but removed it after the March 15 attack.

Bridges added he did not condone any threats made against politicians.

Ardern, who has previously received death threats including from anti-1080 people, said she did not have a "tally" on threats against her, and left it to police or the diplomatic protection squad to handle risks to her safety.