Anti-MMP campaigners say they are disappointed more MPs have not spoken out about their preference to change the voting system.
In the referendum, to be held alongside the November 26 election, voters will be asked whether they want to keep MMP, and which of four systems they would prefer to swap to if MMP was thrown out.
The four other systems are First Past the Post, Single Transferable Vote, Supplementary Member and Preferential Voting.
Prime Minister John Key has previously declared a preference for changing to Supplementary Member, but has remained quiet about the referendum throughout his election campaign.
However, on TVNZ's Breakfast this morning, Mr Key said he would vote against MMP and choose Supplementary Member as his preferred alternative system.
Vote for Change spokesman Jordan Williams, who is heading the campaign to dump MMP, said he was pleased to see Mr Key speak out against the system, and hoped other MPs would follow his lead.
"We're ecstatic that finally there is someone very senior talking about one of the middle options,'' Mr Williams said.
"We have been disappointed that the Prime Minister hasn't led an intelligent debate on this like David Cameron did in the United Kingdom.''
Mr Williams said politicians were focused on the election rather than the referendum.
Keep MMP spokeswoman Sandra Grey said Supplementary Member would deliver First Past the Post-style election results.
"Under MMP you are always sure that your party vote will count even if you don't support the winning candidate in your electorate. Under Supplementary Member you can lose out in a choice of electorate MP and have a devalued party vote to boot,'' she said.
Under Supplementary Member, there are 90 electorate and 30 supplementary seats. The share of supplementary seats a party receives is based on the percentage of the party vote it wins no matter how many electorate seats it has.
One of the major parties would usually have enough seats to govern alone under the system.