They may be lecturing Australians about the need for belt-tightening but the Government's Commission of Audit could not keep to its own budget to produce its landmark stocktake.

The coalition predicted in its midyear Budget update that the commission would spend about $1 million examining the innards of government spending.

But calculations suggest taxpayers coughed up more than double that amount after a small army of bureaucrats were co-opted to produce the five-month study. Publicly available figures show it cost taxpayers about $2.5 million to produce the audit.

It cost $1.9 million for expert staff drafted in from the departments of Finance, Treasury and the Prime Minister and Cabinet to work on the study.

The head of the commission's secretariat, Peter Crone, was paid $157,000 to oversee the probe, while chairman Tony Shepherd was paid $85,000.

Former Howard government minister Amanda Vanstone received $85,000 for her role as a commissioner. Consultants Boston Consulting Group were paid $50,000.