The Greens want to put multi-national accounting firms under the microscope following a serious tax office breach by a former PricewaterhouseCooper employee, saying the incident "may be the tip of an unethical iceberg".
Senator Barbara Pocock is calling on the Albanese government to undertake a review into its use of private accounting firms and the Big Four consulting firms, which the Greens finance spokesperson says "rips the heart out of the public service by poaching good people and are contracted often for hugely inflated fees".
It comes as Peter-John Collins, the former head of international tax for PwC's Australian office, was banned from practising as a tax agent for two years on Monday after he revealed privileged information on the ATO's multinational tax avoidance strategy to clients of the firm.
Mr Collins shared information about the government's plans to other staff at PwC, and that information was then shared to clients.
Senator Pocock said a probe should dig into why the government shared information with the major accounting firm in the first place.
"Sharing details of proposals to reign in tax avoidance with firms like these, whose employees then behave unethically, is like putting the fox in charge of the henhouse," she said in a statement on Tuesday afternoon.
"What confidence can taxpayers have that conflicts of interest like these are not widespread?
The Greens senator added she had asked the Finance Department for the total cost to the public purse but was told in a question on notice it couldn't be separated from other external consultant and contracting costs.
The Canberra Times reported earlier this week the federal government is working to establish a one-stop shop for labour hire and consulting contracts, centralising the process through a single point.
Public Service Minister Katy Gallagher said the move would provide greater transparency over the total cost to the budget and would cut red tape for industry and reduce waste.
The Albanese government during the election promised to restore the public service's capability by reducing its reliance on third-party providers, which had grown considerably under the former Coalition government.
But Senator Pocock said a review should first cover the outsourcing of government work to external consultants.
"They rip the heart out of the public service by poaching good people and are contracted often for hugely inflated fees to undertake policy analysis and run programs that could and should be done much more efficiently by a robust public sector - behaving ethically and without conflicts of interest," she said.
"We need to rebuild the capability of the public service and we need a thorough review of conflict of interest and ethical behaviour processes across the consulting sector where government work is underway.
"These big companies are poaching some of our best and brightest public servants and, in this case, an employee has clearly misused information that they have become privy to in the process of consultation on sensitive government policies."