There is a provision in the Electoral Act 1993 that provides, for some parts of the Electoral Act, a requirement that for amendment to be made, it requires either a 75% majority of Parliament or a majority of voters in a referendum. You can find that at section 268 of the New Zealand Electoral Act.
The things that are covered are the term of Parliament, the makeup of the Representation Commission, the process for determining boundaries, the voting age, and the method of voting. Those are some of the things that are prescribed as requiring a special majority before they can be amended.