Referendum choices: Supplementary Member (SM)

How would the supplementary member (SM) voting system work?
Parliament would have 120 members - 90 of whom would be elected on a First Past the Post basis in single-member electorates. A separate category of MPs would occupy the 30 remaining "supplementary seats".

Voters would have two votes - the first to choose his or her electorate MP; the other for a party contesting the supplementary seats. Those 30 seats and those seats alone would be allocated on a proportional basis. The total number of seats held by each party would be determined by its number of electorate seats and supplementary seats.

Where is SM used?
Also known as "parallel voting", variations of SM are used in more than 20 countries, including Japan, South Korea, Russia, East Timor and Thailand.

What are the advantages of SM?
Those promoting SM argue it blends the best features of true proportional systems and First Past the Post (FPP) which was used as New Zealand's electoral system until 1993 when MMP was adopted by referendum. The principal advantage, according to its advocates, is that SM would produce single-party government and thus stable and effective government. Hypothetically, had the 2008 election been run under SM, National would have won 53 of the 90 electorate seats and 16 of the 30 supplementary seats, giving the party 69 of the 120 seats and John Key an 18-seat majority.

What are the disadvantages?
SM was given serious consideration by the 1986 Royal Commission on the Electoral System, but was rejected because - unlike MMP - it did not go far enough in meeting the commission's objection to New Zealand's then FPP system in terms of fairness regarding the ratio of votes won to seats won.

Again, had the 2008 election been fought under SM, the Greens would have won just two supplementary seats, instead of the nine list seats they got under MMP.

If the threshold was kept at 5 per cent there would have been only three parties in Parliament - National, Labour and the Greens.

Opponents of SM view it as a FPP wolf in sheep's clothing. They see SM as destroying the diversity of an MMP Parliament, while reinforcing Cabinet power and ensuring Labour or National exercise that power unfettered by minor party partners.

That would not necessarily be the case. Japan has SM, but coalition governments almost always involving the conservative Liberal Democratic Party have been the norm.

Some of the alleged faults of SM - principally the lack of true proportionality - could be addressed with a different mix of electorate and supplementary seats. In East Timor, for example, 75 seats are proportionally elected and only 13 members are elected from constituency seats.

Japan's Parliament has 300 electorate MPs and 180 supplementary seat MPs. However, the act setting up the referendum and information produced by the Electoral Commission base their explanations on a 90:30 split of electorate and supplementary seats.

The public debate on the pros and cons of SM will accordingly be dictated by that ratio. SM tends to be favoured by those on the right of politics who want a return to FPP because they believe that produces more effective and decisive governments but think referendum voters will not support a return to the former system.

National's leaning towards SM may also spring from its relative lack of long-term minor party allies under MMP. By the same token, Labour's (understated) backing of MMP may reflect the relative ease that party has had in finding support partners with whom to form a government.

Such political factors may reduce the referendum to a battle between MMP and SM.