New Zealand First and the ACT Party are at loggerheads over whether superannuation will remain affordable for people reaching the age of 65.
The Prime Minister said the government was reviewing the long-term affordability of superannuation and may make some changes, but not drastic ones.
ACT leader David Seymour said forecasts showed the number of people aged 65 and over would double in the next 30 years, meaning changes to superannuation were inevitable.
He said the government should say in the May budget that the age of eligibility would rise over time.
"People actually need clarity, the difference between ACT and National is that we want to put a stake in the ground, and we'd do it this much, you needn't wait for an election campaign for people to find out what their financial future holds in terms of superannuation."
Mr Seymour said ACT's policy was that the age of eligibility should move gradually from 65 to 67.
But New Zealand First said voters were being railroaded into thinking superannuation was unaffordable when the evidence shows that was not true.
Its leader Winston Peters said if people believed superannuation was unaffordable, they should be putting forward the evidence for that.
"What you've got is a very super critical argument being put up to frighten people into thinking that it's not sustainable today, as I say they won't put the evidence out there to justify their position."
Mr Peters said only 3.8 percent of GDP was spent on superannuation, while the OECD average was 7 percent.
He said the New Zealand Superannuation Fund would be $50 billion today, not $33bn if the government had kept up payments into it.
Mr Peters said other parties had been inconsistent on the issue.