The debate over the referendum on the electoral system is heating up, with the anti-MMP lobby group Vote for Change set to endorse the Supplementary Member system.
It comes as the Keep MMP campaign was officially launched in Auckland on Friday.
Up until now, Vote for Change has argued that any system is better than the current Mixed Member Proportional system, but has refused to back an alternative.
On Saturday, it is expected to announce that it favours Supplementary Member.
Vote for Change spokesperson Jordan Williams says there is growing consensus that Supplementary Member is a good compromise between MMP and First Past the Post.
"The great thing about Supplementary Member is that you still have list MPs, albeit fewer of them, but the key difference is that electorates matter.
The trouble with MMP is that if a party loses an electorate, they merely get another list MP. Under SM, electorate seats actually matter and its a lost seat for that party."
Under Supplementary Member, there would be 90 electorate seats and 30 list MPs.
A spokesperson for the Campaign for MMP says with the party vote only applying to the 30 list seats, smaller political parties will suffer.
Sandra Gray says MMP has produced immensely stable government and, as a proportional system, it allows the voice of voters to be heard clearly.
"For third parties, Supplementary Member will see them almost go into obscurity. And I think we've got to recognise one in five voters in New Zealand supports a third party. Those one in five voters deserve a fair chance - and MMP's the one that gives them that."
Ms Gray says the Campaign for MMP aims to spend the next four weeks talking to voters about MMP and how it can be improved.
Insight debate on referendum
A special Insight debate on the electoral referendum featuring Jim Bolger, Michael Cullen, Ruth Richardson and Jeanette Fitzsimons will play just after the 8am news on Radio New Zealand's Sunday Morning programme