No Right Turn has an interesting post (with equally interesting comments) about whether MMP is entrenched in New Zealand, and whether it should be.
The additional question that arises out of the whole discussion is whether anything is really entrenched in New Zealand, given that the clause of the Constitution Act that speaks of some clauses being entrenched (i.e. requiring 75% of parliament or a referendum, rather than a majority of parliament to change or repeal) is not itself among those specified as entrenched.
It is, moreover, quite interesting that “entrenchment” in New Zealand entails a very large supermajority of representatives or a referendum, where the latter means simple majority of votes cast. MMP was initially enacted in a referendum by 53%, which would not have been suficient if the rule were like that used in the recent referenda in British Columbia and Prince Edward Island. In the BC case, the proposed electoral reform (to STV) obtained 57%, but it required 60%* (and it is noteworthy that the super-majority requirement was itself enacted by a simple majority of the povincial parliament).
Of course, the 75% threshold in parliament is unusually high (two thirds is more common), but it reflects the former first-past-the-post electoral system, under which it was not unheard of for one party to obtain three fifths of the seats even without a majority of the vote. While no party ever obtained two thirds of the seats with less than a majority of votes, some came close.
Now that New Zealand has a PR system, and thus such large single-party majorities are impossible (absent a huge majority of the votes) one could wonder whether 75% is any longer necessary, even if one agrees (as I do) that the majority of the day ought not be able to change something so fundamental as the electoral system.
However, perhaps the principle on referenda should be somewhat different. I have expressed my skepticism previously about super-majority requirements in referenda, because they can produce ambiguous results (as the BC case shows).
* And a majority in at least 60% of the districts (which it obtained easily).
h/t Holden Republic