New Zealand will be voting to support the United Nations Global Compact on Migration, Foreign Minister Winston Peters has announced.

The Government had been taking legal advice on the compact, which has been criticised by the National Party as threatening New Zealand's sovereign right to make its own immigration laws - even though it is not legally binding.

The global compact is the first intergovernmental agreement to cover international migration.

It is based on values of state sovereignty, responsibility-sharing, non-discrimination, and human rights, and is aimed at reducing the risks that migrants face while mitigating the factors that keep people from having sustainable livelihoods in their countries of origin.

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Peters said that 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.

Specifically the legal advice states:

• The compact is non-legally binding and does not create legal obligations

• It does not establish customary international law

• It does not give the legal instruments referred to in the text any binding effect that they do not already have in international law

• It reaffirms the sovereign right of states to determine national immigration policy and laws and that states have the sole authority to distinguish between regular and irregular migratory status

• It does not establish any new human rights law, nor create any new categories of migrants, nor establish a right to migrate

• It in no way restricts or curtails established human rights, including the right to freedom of expression

Peters said the advice was important because it was "debunking falsehoods or misguided perceptions".

"New Zealand is voting for the compact because we support greater efforts in controlling migration issues while also being confident our own sovereign decision-making isn't compromised."

National leader Simon Bridges has said the compact treats legal and illegal migration in the same way.

"There is no automatic right to migrate to another country without that country's full agreement, a view which the UN's Global Compact on Migration seeks to counter.

"While not binding, the compact could restrict the ability of future governments to set immigration and foreign policy, and to decide on which migrants are welcome and which aren't."

Speaking on behalf of Peters during Question Time yesterday, Minister David Parker said it was "irresponsible and incorrect" to say that the compact curbs a country's sovereignty.

"The National Party is very lonely in their populist gymnastic," he said, referring to the few countries - including the US and Australia - that have signalled they would vote against it.

Parker further noted that the National-led government had in 2016 voted in favour of a precursor to the compact called the New York Declaration for Refugees and Migrants.

"The reversal by the National Party on its earlier position is desperate, opportunist flip-flop."

The UN General Assembly vote on the compact is expected to begin at 4am NZ time tomorrow morning.