The Electoral Commission says a group wanting to get rid of the Mixed Member Proportional voting system has factually incorrect and misleading information in its campaign material.
The commission has received a complaint about a diagram on the Vote for Change website which says the MMP system requires 120 MPs, while the alternative systems to be voted on in the referendum could work with 99 Parliamentarians.
Chief electoral officer Robert Peden says the Electoral Referendum Act (2010) specifies that each voting system being considered in the referendum will have 120 MPs.
Mr Peden says the group is free to campaign to reduce the number of MPs, but implying that there is a link between voting for a change and having fewer MPs is factually incorrect.
Vote for Change is standing by its campaign material. Spokesperson Jordan Williams says Parliament has already said there couldn't be fewer MPs under MMP because the number of electorates can't be reduced.
Mr Williams says the National Party agreed in 2006 Parliament could be smaller, but that it couldn't be done under MMP.
Meanwhile, a media law expert says Vote for Change realises that many people like the idea of fewer MPs and is downplaying the fact that issue is not being considered in the referendum.
Steven Price, a barrister and law professor at Victoria University, says the group recently added a footnote to its material which says each system would have 120 MPs.
People will vote in a referendum on MMP on election day on 26 November and if more than half choose to keep it, the system will be reviewed. The results will be released in December.
If MMP is rejected, Parliament will decide whether to hold a further 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.