They have no actual constitution. Their “Constitution” is merely the sum total of all of their national laws.
The Constitution of New Zealand is the sum of laws and principles that make up the body politic of the realm. It concerns the relationship between the individual and the state, and the functioning of government. Unlike many other nations, New Zealand has no single constitutional document. The Constitution Act 1986 comprises only a portion of the uncodified constitution, along with a collection of statutes (Acts of Parliament), the Treaty of Waitangi, Orders in Council, letters patent, decisions of the courts and unwritten conventions.