National MP Gerry Brownlee says a new migration pact signed by New Zealand will result in "pretty much open borders".
The Global Compact for Safe, Orderly and Regular Migration is an international agreement to cooperate on migration issues, such as gathering and sharing data, fighting people-trafficking and respecting migrants' human rights.
The non-binding agreement came to life in 2014 in response to massive waves of migrants making their way across Europe, and has been adopted by more than 160 countries.
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"There had to be something done or considered about how those people were going to be treated, so it was a refugee issue," Mr Brownlee told RadioLIVE on Friday.
Though nothing in the agreement legally bounds New Zealand to do anything, Mr Brownlee says it's a "misnomer" to say it's not legally binding.
"I just don't think we should be handing over, even though it's non-binding - and with all due respect, that's a misnomer; there are a whole lot of things that are non-binding as far as UN treaties are concerned, but we still follow them - and to hand over your immigration policy to scrutiny to other UN countries if you don't do what is required - which is pretty much open borders - I think's the wrong thing to do."
Hungary, Austria, Italy, Poland, Slovakia, Chile, Israel, Poland, the US and Australia are among the countries that haven't signed it, even though all - except the US - agreed to it in July, BBC News reports.
Mr Brownlee said Switzerland, one of the countries which sponsored the pact in its early states, hasn't signed up. But Switzerland hasn't ruled out signing it, saying it just needs more time to let lawmakers come to a decision.
The European Commission (EC) says the Global Compact for Safe, Orderly and Regular Migration is not legally binding, as does the New Zealand Government. The EC also says there will be no financial commitment required from any nation, but UN member states will be allowed to contribute "on a voluntary basis" if they wish.
National has promised to withdraw New Zealand from the agreement if it wins the next election.