Under current MMP rules, a political party is entitled to a share of MPs that’s about the same as its share of the party vote if it reaches one of two thresholds (sometimes called clearing one of two "hurdles").
To meet these thresholds or hurdles, a political party must win:
- EITHER at least 5% of the nation-wide party vote;
- OR at least one electorate seat.
A formula – called the Sainte-Laguё formula – is used to determine the total number of seats each party is entitled to in Parliament.
A political party’s total number of seats in Parliament is filled with a mix of Electorate MPs and List MPs.
The Electorate MPs are elected using the First Past the Post voting system (FPP). The candidate who gets the most votes wins. The winning candidate does not have to get more than half the votes.
The rest of a party’s MPs are elected from the party’s list. The number of List MPs each party receives is the difference between a party’s total allocation of seats in Parliament and its number of Electorate MPs.
For example, if a party gets 30% of the party vote it will get about 36 MPs in Parliament (being 30% of 120 seats). So if that party wins 20 electorate seats it will have 16 List MPs in addition to its 20 Electorate MPs.
If a party crosses the 5% party vote threshold but, at the same time, wins no electorate seats, it is still entitled to a share of all the seats in Parliament. For example, if a party wins 10% of the party votes and no electorate seats, all its 12 MPs (10% of 120) will be List MPs elected from the party list in the order they are ranked by the party.
Sometimes a party's share of the party vote entitles it to a number of seats in Parliament that is smaller than the number of its Electorate MPs. When this happens that party will neither have electorate seats taken away from it, nor be allocated any List MPs. Instead, for the life of the Parliament concerned there will be more than 120 MPs in Parliament. This is called an overhang and has happened three times: from 2005-08 there were 121 MPs, in the 2008-11 Parliament there were 122 MPs, and in the 2011-14 Parliament there were 121 MPs.