Prime Minister Kevin Rudd has hinted welfare payments may be means tested as part of this month's federal budget, saying high-income earners don't need government support.

Treasurer Wayne Swan did not rule out introducing means testing in his first budget on May 13 either, drawing a stinging attack from the opposition.

Currently welfare payments such as the baby bonus and childcare support are enjoyed by everyone, regardless of their income.

Mr Rudd refused to put a figure on what he considers a high-income earner.

"It's just that I think once you get right up there, there's the whole question about whether you need support from government," he told the Fairfax Radio Network.

"People struggling with their budget... they are the people who we need to try and help, and they are the people we intend to provide support to through the budget."

Childcare costs

He said he wouldn't go as far as using the term "fat cats", but people at the upper end of the income spectrum did not actually need as much direct support from government.

Families trying to balance the budget on an average income of $50-$60,000 a year needed to be looked after, Mr Rudd said.

"You're looking at putting your kids into childcare and dealing with their childcare costs, dealing with the education costs, having to go to the doctor, particularly in winter, a couple of times a month, and paying the pharmaceuticals, and struggling with your mortgage.

"I mean, they are the people who make up the vast majority of our country who we need to look after."

Mr Swan said he was "not ruling anything in, or anything out" when asked by journalists whether Mr Rudd was indicating welfare payments for high-income earners were under threat.

Opposition treasury spokesman Malcolm Turnbull said Mr Swan was certainly "leaving open" the possibility of means testing the baby bonus, even though Mr Rudd previously ruled that out on March 17.

"Now we see this inexperienced treasurer, this insensitive treasurer, now canvassing or proposing or leaving open the possibility - he's so vague it's hard to know what he means most of the time," Mr Turnbull said.

Baby bonus

"But he's certainly leaving open the possibility of means testing the baby bonus."

For a third day running, Mr Swan hosed down expectations of a huge budget surplus, but said it would tackle current inflation pressures.

"That is why we have got to cut spending, that's why we have got to build a responsible surplus. Otherwise, we will have further price rises, higher inflation, and higher interest rates," he said.

Mr Swan described as "simply inaccurate" speculation the budget surplus could reach a record $21 billion.

"Our fiscal position is this year's budget is dramatically different from what it had been in the past because of the fallout from the sub-prime crisis, because of its impact on share markets, and it will impact upon government revenue," he said.

Opposition finance spokesman Peter Dutton said the government does not yet know what the bottom line of the budget will be because of its "backflipping" and late budget decisions.

"I support a large budget surplus, but it has to be on the back of responsible spending and revenue initiatives, not on the farcical token policy gestures that we have seen to date."