[TMWNN]

As AmericanChopper said, it's mostly irrelevant in the legal sense.

The most important part of stromtium_90's reply is the principle of Parliamentary Sovereignty, the idea that no legislature can do something that a future legislature cannot undo. Every legislature, including the US Congress, follows this principle ... with the exception of the constitution/founding treaty/etc., which is the only thing that is superior to that legislature's power. New Zealand does not have such a document. Neither does the UK. So, theoretically, both countries' parliaments can do anything as long as the action complies with any treaties signed with other nations ... and, of course, the parliaments can always use whatever exit option the treaties provide, if necessary.