Federal Treasurer Wayne Swan has refused to rule out income tax increases as a way of offsetting revenue shortfalls in his budget.

Mr Swan, speaking to ABC Radio on Thursday, was asked four times whether he would increase income taxes in the May 14 budget as claimed by the opposition.

"I'll leave the speculation for everybody else," he said.

"There'll be hundreds, hundreds of stories between now and budget day and most of those stories will be wrong."

When asked whether he could guarantee Australians there would be no increase to income taxes, the treasurer said: "I don't in the lead-up to any budget ... go into that rule-in-rule out routine.

Mr Swan already has conceded a forecast budget surplus is unlikely on the back of revenue writedowns, including a big shortfall in the new mining tax.

Some analysts estimate the deficit for 2012/13 will be between $12-15 billion, leaving the government little room to fund big-spending initiatives such as a national disability insurance scheme and measures recommended by the Gonski school funding review.