The latest Treasury's admission that Australian surplus cannot be supported by current levels of federal revenues only proved that the May budget presented by Treasurer Wayne Swan was grossly unrealistic, the Coalition said.

In a statement, Shadow Treasurer Joe Hockey said the Treasury itself has conceded that the $1.5 billion surplus pledged for delivery by the Gillard Government on 2013 will only be met if new tax programs would be introduced.

In truth, apart from realising savings through spending cuts, the Labor-led government has mapped its surplus goals on the notion that federal finances must be supported by extra tax revenues of about $33 billion within the financial year 2012-2013.

"Australians just cannot trust the government's numbers, now it appears that even the secretary of the Treasury can't believe the numbers either," Mr Hockey was reported by the Australian Associated Press (AAP) as saying on Friday.

He was reacting to the declaration made yesterday by Treasury Secretary Martin Parkinson before a business forum held in Brisbane.

"The days of large surpluses being delivered by buoyant tax receipts are behind us," Mr Parkinson warned.

But the problem, according to Mr Hockey, lies not on the revenue shortfalls but on the manner that the national government has programmed its spending since day one, which he faulted for leaving too big a room for wastes.

Such mistakes can be corrected by what he characterised as the Coalition's 'fresh approach', which will be underpinned by the setting up of a Commission of Audit.

The same body, Mr Hockey said, will be tasked to "find waste in the Commonwealth's $364 billion of annual expenditure."

As a Treasurer, the Liberal frontbencher vowed that he will "make cutting the waste a top priority."

It appears, however, that Mr Hockey had completely missed the whole point of what Mr Parkinson had aimed to convey when he delivered his remarks, a statement sent out by the office of Mr Swan said.

The Treasury chief had meant to press on the fact that "the rivers of gold that (former Liberal treasurer) Peter Costello relied on were a bubble and have simply stopped," the statement said.

A spokesman for the Treasurer reminded Mr Hockey that facts would show that government expenditure ratio has been set for the current financial year at below 24 per cent of the gross domestic product (GDP) and will remain the same over the next four years.

Mr Swan's statement also questioned the wisdom of introducing an audit commission which could hurt the prospect of employment security and creation by "slashing thousands of jobs and cutting funding to community groups."

The same statement expressed concern that the Coalition would want to keep such reality under wraps until the election is over and done with, which exactly was the tactic employed by the Liberal-National alliance in Queensland.

Mr Swan's spokesman suggested too that Mr Hockey would do better to first address the questions created on the way the Coalition had so far manage its finances.

"No one can take Mr Hockey's comments seriously until he outlines how he'd fill the Coalition's $70 billion budget black hole," the Treasurer's statement said.