New Zealand won't be joining Australia in refusing to sign a non-binding United Nations migration agreement and has called claims the deal would undermine its national sovereignty "falsehoods".

The Morrison government last month said it would not be signing the UN Global Compact on Migration, despite having been actively negotiating the wording for two years, because it would compromise Australia's border security and immigration settings.

In a decision announced on Wednesday afternoon, New Zealand Foreign Minister Winston Peters said his country would be supporting the deal and that legal advice to his administration showed the agreement was "neither legally binding nor constraining on this country setting its own migration policies".

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"The government would not support the UN compact if it compromised New Zealand's sovereignty or could in any way take precedence over our immigration or domestic laws. But the compact does not do that," he said in a statement that did not comment on the Australian decision.

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"The legal advice is not surprising but is important advice in debunking falsehoods or misguided perceptions being spread about the implications of this framework."

While the compact was officially adopted by 164 governments last week, New Zealand's government has been mulling its options amid lobbying from across the political spectrum.

The country's conservative opposition National Party has been calling for ministers to refuse, making similar arguments to Mr Morrison, and also criticising what he said was "uming-and-ahing" by officials, despite having a final draft since July.

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On Wednesday, its Foreign Affairs spokesman Todd McClay, said the party would withdraw from the deal when in office and argued other non-binding UN declarations - such as the UN Declaration on the Rights of Indigenous Peoples - already affected domestic laws.

"By supporting it the government has also indicated their intention to act consistently with it," Mr McClay said.

Mr Peters' NZ First party, sometimes described as populist, forms part of the coalition government and campaigned on heavy cuts to immigration last year.

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Other countries including the United States, Hungary, the Czech Republic, Slovakia, Israel, Poland and Bulgaria have also refused to sign the pact, which is described as "the first ever global framework on international migration".

It includes a list of 23 objectives for "safe, orderly and regular migration". The UN has insisted the pact was never intended to be legally binding and does not establish any new right to migrate.