How MMP works in New Zealand, and why your party vote matters most
You get two votes and they do very different things. One of them decides the shape of Parliament; the other mostly doesn't.
Your two votes
On election day you fill in two things. The party vote picks which party you want in government. The electorate vote picks which person represents your local area.
Almost everyone assumes these carry equal weight. They don't. The party vote is the one that decides how many seats each party gets in Parliament, and therefore who can form a government.
Parliament has 120 seats. 71 come from electorates (64 general, 7 Māori) and the remaining 49 are filled from party lists to make each party's total match its share of the party vote.
Why the party vote does the heavy lifting
If a party wins 30% of the party vote, it gets roughly 30% of the seats, about 36, no matter how many electorates it wins. If it won 20 electorates, it gets 16 list MPs on top. If it won 36 electorates, it gets no list MPs.
So your electorate vote changes *who* represents your area, but rarely changes the balance of power. Your party vote changes the balance of power directly.
The 5% threshold, and the electorate exception
A party needs either 5% of the party vote or one electorate seat to get into Parliament. Miss both and its votes are redistributed among the parties that made it.
This is why an electorate race can matter far beyond its own boundaries: a party polling 3% that wins one seat brings its whole party-vote share into Parliament with it.
Nobody wins outright, so coalitions decide the government
Under MMP a single party almost never wins a majority. The government is formed afterwards, by negotiation between parties.
That means the party you match with most closely may end up governing with a party you match with poorly, or may sit in opposition despite a good result. It's a real limitation of any voting quiz, including this one: we can tell you who matches your views, not who will end up in Cabinet.