The Prime Minister, John Key, says the National Party can work under any voting system, and it will not be campaigning for any particular system in the lead-up to this year's referendum.
The non-binding referendum will be held alongside the general election in November.
It will ask whether voters want to retain MMP or change to another system, and then it will give four other voting systems to choose from.
If a majority of voters want to change the system, a second non-binding referendum will be held in 2014 with a choice between MMP and the preferred alternative.
Mr Key says MMP has settled down and has some real advantages.
"Proportional voting systems allow greater ethnic diversity and a better gender balance. It also brings in people who, because of their skill set and timeframe, may not have been able to get into parliament, people like Tim Groser and Don Brash, who have come into parliament through the list system."