MMP is still clinging to its lead in the referendum race.

A Herald DigiPoll survey showed 47.3 per cent of people would vote to keep the electoral system, while 41.9 per cent said they would vote for a change.

Last week's poll indicated a slight thinning of support for MMP, 43.3 per cent voting to keep it and 39.6 per cent to drop it, but polls have continually shown a voter preference to stick with the status quo.

Keep MMP campaigner Sandra Grey said the results reflected what her group was hearing on the ground, and that there was no overwhelming desire for change.

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As well as whether or not voters want to retain MMP, the referendum will ask them to choose between four options that could be a replacement for the current system.

The four choices are Single Transferable Vote, Supplementary Member, First Past the Post and Preferential Voting.

A review of the system is to be conducted if voters choose to retain MMP.