In a Christmas email, New Zealand First has slammed its opponents for stirring up an "emotional debate" about the controversial United Nations (UN) Global Migration Compact.
On Wednesday, Foreign Affairs Minister Winston Peters announced New Zealand would support the UN Global Compact for Safe, Orderly and Regular Migration.
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The agreement creates non-legally binding conditions for countries to "enable all migrants to enrich our societies through their human, economic and social capacities," according to the UN.
Mr Peters made the announcement after taking legal advice from Crown Law confirming New Zealand's sovereignty would not be compromised by adopting the agreement.
But the National party has been vehement in their opposition to the Compact, criticising it as potentially leading to an undermining of national sovereignty, and saying it would be reversed under a National government.
In an email on Friday, NZ First responded saying its "political adversaries" will be "telling everybody that they're going to 'overturn' the UN Migration Compact and make various inflammatory claims that the 'Compact' is going to permit mass migration into New Zealand".
Earlier this month, National leader Simon Bridges said: "There is no automatic right to migrate to another country without that country's full agreement, a view which the United Nation's Global Compact on Migration, set to be signed next week, seeks to counter".
The NZ First email, with the subject line "they are not telling the truth", said the Compact wouldn't lead to increased migration and "already immigration has fallen significantly and will go on falling".
According to Statistics NZ, in the year to the end of October net gain in migrants was 61,800, down 8900 from the October 2017 year.
NZ First has long campaigned on reducing immigration numbers, including wanting to decrease migration to 10,000 a year in 2017.
The party's Facebook page has been inundated with users criticising its decision to support an agreement they believe will lead to New Zealand's migration settings being undermined.
"I voted for you, I encouraged my family & friends to as well, because I thought you were the best option, but you did the opposite of what your constituents wanted, you betrayed your own people Winston, your own voters, not cool," said one user.
But the party is confident the agreement will have "absolutely no legal effect in New Zealand and most importantly make absolutely no change to New Zealand First's principled approach to immigration" into New Zealand.
"This has become an emotional debate based on certain political interests published a misconceived version of the Compact and its impact on New Zealand."
While emphasising the need for international cooperation, the Compact explicitly recognises the sovereignty of individual nations.
"It fosters international cooperation among all relevant actions on migration, acknowledging that no State can address migration alone, and upholds the sovereignty of States and their obligations under international law," the Compact says.
Another point of contention about the Compact is where it states countries should promote independent and objective reporting "by sensitising and educating media professionals on migration-related issues and terminology".
However, the Compact also notes it is committed to protecting "freedom of expression in accordance with international law, recognising that an open and free debate contributes to a comprehensive understanding of all aspects of migration".
On Friday, Gerry Brownlee said signing up the agreement wasn't a good move.
He said 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."
The decision to develop a compact was first made by UN Member States, including New Zealand, in September 2016. The process towards it began in April 2017, stewarded by representatives from Mexico and Switzerland.
After months of negotiations, the final draft of the agreement was decided upon in July.
"In the end, New Zealand will be voting for a cooperation framework that was clearly set out at the start of the Compact's negotiations process in 2016 when the New York Declaration for Refugees and Migrants was unanimously adopted by all UN member states, including New Zealand under the previous government," said Mr Peters.
National's foreign affairs spokesperson Todd McClay told Newshub last month that while in Government National had "decided against joining the UN Declaration on Refugees and Migration, and declined to attend a forum at the UN on migration."
Similarly to Mr Peters, the UN says that the Declaration received unanimous support. In November, Newshub attempted to clarify National's supposed opposition to it in 2016, but received no response.
Green Party Foreign Affairs spokesperson Golriz Ghahramn told Newshub signing up to the Compact was extremely important.
"It is paramount that New Zealand, a responsible international citizen, be part of the cooperative solutions initiated by the Compact."