The government has admitted it might not get the budget back into surplus by 2014/15.

Finance Minister Bill English says the target is still 2014/15 but it's becoming an increasingly difficult challenge.

"If we can get to near surplus, somewhere around there, I think that'll be an achievement," he told reporters on Thursday.

"It's about determination and direction - in an economic sense, a few hundred millions here or there in a $70 billion budget is unimportant in the long term."

Earlier Mr English told a business audience in Wellington that slower global growth, dwindling revenue from state-owned enterprises and rising finance and earthquake costs had eroded the government's finances.

The outlook had deteriorated by about $1 billion compared with recent forecasts, he said.

"Returning to surplus by 2014/15 is a big challenge, it will require tight control over spending for the foreseeable future," he said.

Mr English says he's committed to delivering a zero budget on May 24.

"It will target spending to ensure we invest in priority areas like health, education science and innovation and improving incentives around work and welfare," he said.

Mr English has previously said increased spending in those areas will have to come from cuts elsewhere.

The government will introduce a spending cap, as part of its supply and confidence agreement with the ACT Party, he said on Thursday.

The proposed legislation will mean governments will have to restrict spending increases to population growth and inflation.

Spending on natural disasters, finance costs and unemployment benefits will be exempted.

If an administration breaches the cap, it would have to explain why and outline how it will get back within the cap.