A National-led government is promising to hold a referendum on MMP no later than 2011.
But party leader John Key today said if voters decided to ditch MMP a second referendum would have to be held to decide which system should replace it.
That meant any change was unlikely until the 2014 election or "possibly later".
Mr Key, who made the referendum one of 10 election pledges in his speech to National's annual conference today, said he believed voters would reject MMP.
"I think the country may well vote MMP out but I think they will vote in another proportional system," he told reporters.
"I don't think they'll go back to first past the post."
But after 12 years of MMP it was important to give voters a choice, he said.
"I do think voters thought they were going to get an opportunity to kick the tyres and we're giving them that opportunity."
Mr Key outlined 10 election pledges in his speech to his party's annual conference.
They are to:
1. deliver an ongoing programme of tax cuts;
2. take a more disciplined approach to government spending;
3. cap the number of bureaucrats;
4. crackdown on gangs and the methamphetamine trade;
5. reform the Resource Management Act in its first 100 days of government;
6. open some government services up to private sector management;
7. set national literacy and numeracy standards for primary school children;
8. retain all existing superannuation entitlements and grow them through tax cuts, with Mr Key promising to resign if the pledge is not kept;
9. repeal the Electoral Finance Act but retain provisions which provide for greater transparency in political donations;
10. hold a referendum on MMP no later than 2011.
National leader John Key has promised to spend an extra $5 billion on infrastructure by 2014, but Labour has labelled its plans to borrow to pay for it as "reckless".
Mr Key today outlined a bold plan to jump start New Zealand's faltering economy by fast-tracking the planning and building of key infrastructure projects such as roads.
But his plan to pay for it by lifting Crown debt by about 10 percent has opened up a key election battleground, with Prime Minister Helen Clark accusing National of taking risks with the economy.
Mr Key's infrastructure salvo follows his announcement to National's annual conference yesterday the party would bring forward a second round of tax cuts to April next year and deliver a third in April 2010.
National would not borrow for its bigger, faster tax cuts, but Mr Key signalled cuts in other areas were likely.
There would be "modest" changes to KiwiSaver and National would alter the shape of the second and third rounds of Labour's promised tax cuts.