Greens spokesman David Shoebridge said the consultancy firms had targeted the department as a source of "almost unlimited growth".

"What is really remarkable is how the big four consultants' feasting on defence contracts has surged in the last six years, almost doubling year-on-year," he said.

"With almost daily scandals now breaking where consultants have breached secrecy requirements when it suits them, there is a major national security risk with this."

University of Wollongong senior lecturer Erin Twyford said the reliance on consultants had demonstrated a "hollowing out" of the public sector.

Dr Twyford said the use of consultants was a method to avoid personal liability for public servants accused of giving bad advice or misconduct. 

She warned the big four firms shouldn't be allowed to become "too powerful" in being allowed to help devise a new regulatory framework.

Senator Shoebridge said Defence had "no credible answers" to how it was managing risks, apart from asking for more self-reporting from consultants.

There have been allegations KPMG billed the Defence department for hours never worked.

KPMG reportedly charged the department $1.8 billion over the past decade.

The firm has said it could not find evidence to support claims of improper billing of Defence.

PwC has been embroiled in a scandal over its use of confidential government information to help clients avoid tax.

Assistant Minister for Defence Matt Thistlethwaite said the government was keen to use consultants less and the public service more.

He said it was unfortunate there had been a "default" approach in the past of immediately going to a contractor to sort out problems in government, despite having a "highly skilled, capable public service".

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