Mr Williams said many people had high hopes that MMP would create a new era of consensus politics but instead "small groups and party bosses can now hold the rest to ransom".
The Vote for Change Society today released a list of high-profile supporters including former Wellington mayor Kerry Prendergast, former Waitakere mayor Bob Harvey, businessman Peter Shirtcliffe who ran a campaign against MMP before it was introduced in 1996, public relations specialist Anna Hamilton-Manns and political scientist Emma Daken.
A pro-MMP lobby group, Campaign for MMP, has been active since the Government announced the referendum shortly after the 2008 election.
Recent opinion polls have indicated a majority of voters want to keep MMP.
Labour leader Phil Goff said today he didn't agree with the Vote for Change campaigners.
"I agree with the principle that every vote should be of equal value, and that's something MMP achieves," he told reporters.
"That's why personally I will be backing MMP...if a party gets 20 per cent of the vote, surely it should get 20 per cent of the representation in Parliament."
Mr Goff said the decision was for New Zealanders to make, not the politicians.
"But I think you will find that most people in the Labour Party think that MMP is a fair system based on that no matter where you live your vote does count, and is of equal value to everybody else, and I think that's pretty fundamental to democracy."
- NZPA