Prime Minister John Key is planning a softening-up exercise tomorrow with a major pre-Budget speech addressing potentially unpopular changes to the highly popular KiwiSaver scheme.

Mr Key said yesterday that he did not believe the Budget changes would break election promises.

But he hinted that Government contributions could be limited until its books return to surplus.

The scheme has 1.67 million members and in the current financial year the Government will pay about $922.5 million total contribution, including the $1000 it pays to kick-start each new account.

Potential areas for change are cutting or reducing the Government's own contribution to KiwiSaver accounts of up to $20 a week and up to $1040 a year. It could drop the $1000 kick-start contribution to new accounts and/or target Government contributions to low-paid savers.

National campaigned in 2008 on three changes to cut the minimum rate for employees and employers from 4 per cent of gross salary to 2 per cent; to cut the tax credit paid to employers whose staff were enrolled in the scheme; and to make some changes relating to discrimination against non-members of the scheme in a workplace.

But beyond those three changes National promised that "KiwiSaver members will keep their current KiwiSaver entitlements".

To keep its promise, National's new changes would not take effect until after it had received a mandate at the next election.

The Government has foreshadowed changes to KiwiSaver as well as Working for Families where some wealthy families rearrange their finances to qualify for state assistance, and to repayment of student debt, though interest-free loans will remain.

Mr Key said yesterday that they were "big schemes put in place in different economic conditions by the previous Labour Government".

"We are all aware about how those conditions have changed but we also recognise that New Zealanders value interest-free student loans, KiwiSaver and Working for Families and what we want to do is make sure they are there for the long haul in a form that is affordable, recognising that there are fluctuations in whether the Government is in surplus or in deficit."

He said that where the Government was borrowing for savings, that was not lifting national savings.

It is an argument the Government used in the past about dropping contributions to the Cullen super fund until the books returned to surplus.

Neither Mr Key nor Finance Minister Bill English have used it in reference to Government contributions to KiwiSaver.

Labour leader Phil Goff said that Mr Key should not have made promises he could not keep about KiwiSaver entitlements.