The Campaign for MMP group says MMP is the fairest system on offer at this year's referendum on the electoral system.
General election voters on 26 November will be asked whether New Zealand should retain the Mixed Member Proportional system.
New Zealanders voted by 54% to 46% in 1993 to ditch First Past the Post and replace it with the Mixed Member Proportional system.
Former Prime Minister Jim Bolger has thrown his weight behind one of four alternative systems, Supplementary Member.
He says MMP has transferred power from voters to the political parties.
Mr Bolger backs Supplementary Member, where the party vote would count towards only a share of 30 list MPs, with the remaining 90 electorate seats decided by First Past the Post.
However, the Campaign for MMP says Supplementary Member is not a proportional system.
Listen to more on the referendum on the electoral system in Insight.
