Minor party mayhem is turning voters off MMP, a poll released tonight shows.

The TV3 survey asked voters whether they were more or less likely to vote to keep MMP in next year's referendum after the Act Party's recent leadership ructions and the resignation of David Garrett over the passport fraud scandal.

It showed 32 per cent, one in three, were now less likely to vote in favour of MMP with 13.7 per cent more likely to support it and 49.1 per cent saying it made no difference.

The poll also questioned voters about ACT leader Rodney Hide, and just over 71 per cent said he wasn't fit to continue to lead the party because he had known Mr Garrett stole a dead infant's identity to get a false passport 26 years ago and covered it up.

Mr Hide said that given the media frenzy over ACT's leadership issue and Mr Garrett he was surprised the figure wasn't higher.

Prime Minister John Key said today he thought problems within small parties was likely to undermine confidence in MMP, a system which allows them to hold seats in Parliament, but that didn't necessarily mean people would vote to change it.

A referendum will be held at the same time as next year's general election asking voters whether they want to keep MMP or change to another system.

They will also be asked to choose a preference for an alternative system from a list of options.

If a majority want to change, a second referendum will be held in 2014 when MMP will be run off against the preferred option from the first referendum.