Prime Minister Kevin Rudd is maintaining pressure on the opposition, reiterating his claim coalition senators are mounting a $6 billion raid on Labor's first budget.

Already, the Senate has knocked back three budget measures - the $555 million luxury car tax increase, a $23 million contribution from drug companies to the cost of the Pharmaceutical Benefits Scheme (PBS), and closing a million-dollar tax loophole for family trusts.

Under threat are the government's plans to increase the income thresholds at which the Medicare surcharge levy applies to taxpayers without private health insurance, a tax hike on alcopops, and scrapping an excise exemption on condensate, a light crude oil developed from natural gas.

Mr Rudd says the coalition's position is wrong, especially during a time of global economic uncertainty.

"The cornerstone of responsible economic management is to have a strong surplus," he told the Seven Network today.

"We brought one down in the (May) budget - $22 billion - because we need that as a buffer for the future.

"Now the Liberals are conducting a $6 billion raid on it and it is a real problem."

If the opposition was successful it would put upward pressure on inflation and interest rates, Mr Rudd said.

"Having just got the first interest rate cut in seven years, that is just exactly the wrong way for the Liberals to go."