An anti-MMP group says the Labour Party leader has used the flaws of the voting system to suit his own political purposes, which the group finds horrifying.
On election day, voters will be asked in a referendum whether they want to keep MMP, in which case it will be reviewed, or choose another voting system.
Phil Goff said on Saturday that if MMP is retained and Labour elected to government, it would get rid of the ability to 'rort' the system.
Mr Goff was referring to Friday's informal meeting in a cafe between National Party leader John Key and the ACT's Epsom candidate John Banks, widely seen as a signal to National supporters they should give their electorate vote to Mr Banks.
Jordan Williams, the spokesperson for Vote for Change, says Mr Goff's comments about Labour changing MMP show the very problem inherent in keeping it at all.
"Of course, politicians are going to want a voting system that suits themselves. That's why changes to our electoral system should be put to the public and referendums, rather than handing that power over to the politicians. And the only way to get that is by ticking change."
The referendum will be held on election day on 26 November. If more than half choose to keep MMP, the system will be reviewed. The results will released in December.
If MMP is rejected, Parliament will decide whether to hold another referendum in 2014 to choose between MMP and the most popular alternative system.
The other choices are: First Past the Post, Preferential Voting, Single Transferrable Vote and Supplementary Member.