In 1947, New Zealand adopted the 1931 Statute of Westminster after years of opposing and ignoring the Statute. New Zealand had plenty of practical and cultural reasons to oppose the Statute and its adoption - one that came up, albeit very briefly, was the Treaty of Waitangi.
During the second reading debate on the adoption of the Statute of Westminster, Labour MP and later Attorney-General Dr Martyn Finlay, made a statement that the Treaty wasn’t affected by the adoption of the Statute. In the Hansard (page 548) an interjection questioned this position:
Dr Martin Finlay “It will not affect in any way at all the Treaty of Waitangi; it will have no effect whatsoever on that historic document”
Mr Sutherland “There is some doubt about that”
“There is no doubt about it at all.”
Curiously this issue wasn’t explored further. It does raise the question as to what Mr Sutherland (Andrew Sutherland, MP for Hauraki) was referring to. Today, it’s clear that the creation of a separate “New Zealand Crown” that eventually resulted from the adoption of the Statute (more on that later) would have an affect on the Treaty. If not practically, then at least in legal theory. Symbolically the Treaty was first signed in 1840 between representatives of the British Crown and Māori iwi.
A change to the “British Crown” component of that equation by splitting off a separate New Zealand Crown would clearly have an affect, although perhaps not a breach of the Treaty. The issue is a question of consent or consultation.
Arguably the splitting of the Crowns wasn’t to happen until the 1973 amendment to the New Zealand Constitution Act, when the General Assembly (Parliament) asserted extra-territorial powet to legislate and retrospectively validated any legislation that had done so prior to the amendment Act. But without the Statute of Westminster Adoption Act 1947, that wouldn’t have happened.