A vote for MMP at next year's referendum will effectively be a vote for a thorough review of the system.
The Electoral Referendum Bill - reported back from select committee yesterday - recommends a review to take place, but only if a majority vote to keep it in the referendum.
Labour and the Greens have pledged to fight to have MMP reviewed regardless of the referendum result, which would mean that a referendum in 2014 would pit a potentially improved MMP - rather than the present system - against an alternative.
The Government has promised a referendum on MMP next year at the same time as the general election.
If a majority want a change, another referendum will be held in 2014 asking voters to choose between MMP and the most popular of four alternatives: First Past the Post, Preferential Voting, Single Transferable Vote or Supplementary Member (SM).
One of the main changes to the bill is a $300,000 limit on advertising spending, bringing it into line with the proposed third-party advertising limit for general elections.
The original bill had no limit, but the committee - except the Act Party - said a limit would "level the playing field" and prevent undue influence on the referendum result by the wealthy.
Justice Minister Simon Power rejected the view that MMP should be reviewed regardless of the referendum result.
"The key to the public being able to make a choice, if a second referendum is triggered, is that they need to make a choice against a system that they've seen in operation," Mr Power said.
The bill specifies the aspects of MMP that would be considered in a review, including the thresholds for a party to get into Parliament.
The system has been criticised because a vote for a party that falls short of 5 per cent - such as New Zealand First last election - is a wasted vote.
Another criticism has been that the Act Party has five seats in Parliament - courtesy of Rodney Hide's hold in Epsom - even though it had fewer party votes than New Zealand First.
A review would also look at the party list. Now, electorates can reject candidates who then ride into Parliament on the list, such as Cabinet minister Chris Finlayson or Labour Party front-benchers Charles Chauvel and Maryan Street.
Excluded from the review would be the Maori seats or the number of MPs.
Mr Power said a review of MMP would include input from the public.
The committee detailed a 90/30 split for the SM system, in line with the 1986 Royal Commission report.
This would mean under SM, 90 MPs would be selected from electorates, and the remaining 30 would come from the party lists.
This would be less proportional than MMP, which has a 70/50 split.
Campaign for MMP spokeswoman Sandra Grey said people could now vote for MMP knowing that it was a vote for a review.
"MMP has been running well for five elections, but that doesn't mean it can't be made even better."
MMP REFERENDUM 2011
* Will ask if voters want to keep MMP, and to select one of four alternatives.
* If more than 50 per cent want to change, a referendum in 2014 will ask voters to choose between MMP and the most popular alternative.
* Advertising limit of $300,000 for the referendum period.
* If more than 50 per cent want to keep MMP, it will be independently reviewed.
WHAT WOULD BE REVIEWED?
* The thresholds for getting into Parliament (5 per cent of the vote or winning an electorate seat).
* That a candidate can stand in an electorate and be on the party list, which voters can't control.
* That a party (such as the Maori Party) can have more electorate seats in Parliament than the number of seats proportional to its share of the vote.