I'm not supporting national (I am politically agnostic and do not support any party). But we're not really discussing this at any real level of details. No one is getting into the weeds here it is just high level nonsense. No one is addressing the expat component of this. And quite frankly I can't fully reconcile why expats should be treated any differently to an immigrant. You choose to leave the country then there is an argument that you should be placed on the same level as someone else wanting to get in. Just because you were born in NZ it means you have special entitlement? Hmm. Can't say that is fully logical.
You can't just make citizens stateless because they leave the country. It goes against Article 15 of the United Nations Declaration of Human Rights.
The government would be within their right to disallow dual citizenship, however, and many countries exercise this right. This would mean that any citizen of New Zealand that acquired a second nationality would automatically lose their New Zealand citizenship. It would also mean that anyone seeking to acquire citizenship of New Zealand renounce their citizenship of any other countries.
The practical problem of this is that a lot of people seem to hold both Australian and New Zealand citizenship a large enough number as to make changing this law difficult politically. If you force someone to renounce their citizenship of another country, be prepared for that person never to vote for you again.