The Supplementary Member electoral system is shaping up as one of the main contenders to replace the current voting system if voters opt to ditch MMP at this year's referendum.
While there is little polling data on which of the four alternative systems might find favour with the public, Supplementary Member is attracting support from a number of high-profile critics of MMP, including former prime minister Jim Bolger.
Radio New Zealand political reporter Julian Robins takes a look at Supplementary Member.
Of the five systems being considered in the referendum on 26 November, only two are considered fully proportional - MMP and Single Transferable Vote.
Another two, First Past the Post (FPP) and Preferential Voting, are what's known as majoritarian - that is, a single party tends to win a clear majority and govern alone.
Supplementary member lies somewhere in-between.
Michael Bassett, a former Cabinet Minister in the fourth Labour Government, is throwing his weight behind the system.
"There is an argument for a supplementary list. There are a lot of people out there in the community who don't want to go through the hurly-burly of electorate politics, particularly during an election year. They have something to contribute; would willingly contribute it and if you're going to have that supplementary list elected, then the easiest way to do it is with the second vote."
So, just as they do currently under MMP, voters under Supplementary Member would have two votes.
There would be 90 electorates, with the candidate who gets the most votes winning. And, unlike MMP where the party vote decides the overall make-up of Parliament, the party vote would only apply to the remaining 30 list MPs.
Jordan Williams, spokesperson for anti-MMP lobby group Vote For Change, says Supplementary Member would make for more decisive elections.
"It's whether you think that democracy is about representation, or do you think that it's about accountability. Because unfortunately, there is a bit of a trade-off there. That's why I like SM, in that you get your accountability to electorates - but also you have a proportion of the House that is list MPs, and so you get that representation for minority groups."
Supplementary Member would strengthen the position of the major parties and almost certainly reduce the size of the minor parties.
A 10% party vote would translate into three list MPs, over and above any electorate seats a party wins.
Under MMP, that same party vote would translate into about 12 seats in the House - regardless of how many electorates a party wins.
Political scientist Therese Arseneau says Supplementary Member would usually result in single-party government.
"People sometimes call Supplementary Member semi-proportional. I see Supplementary Member - at least the one that's proposed here - it's basically three quarters a First Past the Post system with one quarter a party list top-up system that is based on proportionality."
Supplementary Member is currently used in more than a dozen countries around the world, including Japan, South Korea and Thailand.
