Key Points:

National leader John Key says if a public referendum calls for MMP to go, he does not favour a return to First Past the Post but is "leaning towards" another proportional system.

Mr Key has announced National will hold a binding referendum on the future of MMP if his party becomes government.

He said his personal preference if the system was dumped would be for Supplementary Member. Under SM, electorate MPs make up the bulk of Parliament and the remainder of seats are allocated proportionately.

"I'm leaning towards Supplementary Member," Mr Key said. "It allows for proportionality while ensuring it's not the dominating factor. You get the best of MMP without it being overpowering. That is the reason why."

He said National wanted a binding referendum because there was a strong public call by voters to pass their verdicts on MMP. He did not believe that necessarily translated to a desire to dump MMP, on which opinion was "mixed".

He said another possibility was that a referendum would call for changes to the MMP system rather than wholesale change of the voting system.

The National proposal is a two-stage referendum, similar to those held which decided the change to MMP in 1992 and 1993.

Peter Shirtcliffe, one of the key anti-MMP campaigners in 1993, also believed SM should be among options for voters. However, he said the first issue was to get a referendum, which he believed should be held in one go.

The SM system would favour National and Labour, which hold the majority of electorate seats but would also get their proportion of the party vote as well. However, voters could learn to vote strategically with the party vote.


SUPPLEMENTARY MEMBER

* Electorate MPs are voted in as normal and Parliament is "topped up" by a further tranche of seats allocated on a proportional basis.

* Unlike MMP, in which the party vote determines the shape of all 120 seats in Parliament, under SM, the party vote only applies to the top-up seats.

* If our current number of MPs and electorates was retained, it would mean on top of the 70 electorate seats, a further 50 would be allocated according to the party vote.

* Was described as having "real merit" by the 1988 Royal Commission which eventually recommended MMP.