Opposition parties want a cap on how much lobby groups and other third parties can spend on political campaigns before an election.
The Government has issued proposals for a re-write of the electoral laws.
Third parties would have to identify themselves if they ran a campaign costing more than $12,000, but there would be no limit on what they could spend.
The Green Party says without a cap, the electoral system will be left wide open to groups with money trying to influence the election result.
And the Labour Party says having no spending cap risks the free-for-all on election spending seen in the United States.
But anti-MMP campaigner Peter Shirtcliffe says there's no need for spending caps as long as the public knows who's funding the campaigns.
Voters will be given a choice of whether or not to retain the present voting system at next year's general election.
They will then be asked to consider four other options: first past the post, preferential voting, single transferable voting and the supplementary member system.
If they vote for change, a second referendum will be held in 2014 pitting MMP against the most popular alternative electoral system.
