"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.
"National will not be supporting this agreement and we will reverse the decision if this Government signs up to it."
Immigration Minister Iain Lees-Galloway and Foreign Minister Winston Peters are considering advice on the compact.
Peters said while it was not a binding agreement, there could eventually be pressure to treat it as binding.
"The only concern we would have is whether or not we were compromising this country's sovereignty.
"In terms of morale and in terms of integrity, non-binding sometimes means binding. Why would a first-world country in a democracy sign something with no intention of abiding by it?"
Lees-Galloway said there were important values behind the compact, and did not understand National's position, given that the compact is not legally binding.
"The reason for National's opposition is quite unclear at this stage. It doesn't restrict New Zealand in any way. We will continue to set out own immigration laws."
He said the text of the compact was "sound", but there were aspects New Zealand disagreed with, such as having identity cards for migrants, and what could be viewed as regulation of free speech.
He said other countries that had similar concerns - including Sweden, Canada, the UK, France, and Germany - had indicated they would sign up.
National's foreign affairs spokesman Todd McClay said there might eventually be pressure to adopt parts of the compact in domestic legislation.
"It's not for the UN to tell NZ what to do."
