A federal senator says reports of a former Deloitte partner leaking confidential defence documents demonstrate the "threat" consulting firms pose to Australia.

Former Deloitte consultant David Milo used confidential government documents to try and win work for his new consulting firm Synergy 360, according to Nine News reports on Thursday.

In 2018, a year after leaving Deloitte, Mr Milo reportedly sent private military documents to Synergy 360 colleagues who were trying to win major military contracts at the time.

In a statement, a spokesperson for Deloitte said it had made Defence aware of the matter.

"It appears shortly before he left the firm David Milo transferred information from his Deloitte email to his personal email. Deloitte had no knowledge of this activity nor of these emails," the statement read.

"We take this discovery extremely seriously as it represents a breach of our policies and Mr Milo's obligations."

The company said it would conduct a thorough investigation, and demanded Mr Milo return "Deloitte information", though it said the information was not classified and held no commercial advantage.

Deloitte said systems installed since Mr Milo left would be able to detect a breach of a similar nature in "real-time", enabling immediate action.

The documents, seen by Nine News, reportedly relate to work Mr Milo did on ForceNet — a Department of Defence communication tool that Deloitte was paid millions to develop.

The reports come just days after a Senate committee grilled Deloitte executives about the way they handled complaints of misconduct.

Labor senator Deborah O'Neil, who sits on the committee, told the ABC the reports about Mr Milo, if proven, were an "outrageous betrayal" of the public's trust.

"This is the day that Australians see, with absolute clarity, that unethical consultants are a threat to the nation," she said.

"Consultancies are profit-driven entities and employ many individuals who are not bound by any professional code or regulatory restraint or punishment."

Senator O'Neil said she would urge the committee to recall Deloitte to give evidence about this "deeply concerning" matter.

The parliamentary inquiry was launched in response to what has become known as the "PwC tax scandal", which involved senior partners of consulting firm PwC misusing confidential government information.

The information related to government plans to tax multinational companies and the partners made millions by advising said companies about how to avoid that tax.

The scandal pushed PwC to sell off the government consulting arm of its business to equity firm Allegro.

The federal government has acknowledged an over-reliance on the big four consulting firms, which include PwC and Deloitte, and has committed to slashing its external consultancy spend by $2 billion.

In the past decade, Australia's state and federal governments have forked out more than $10 billion on the big four consulting firms.

This week's senate committee heard Deloitte earned $712 million in government contracts last year. This made up 25 per cent of its total earnings.

Last month, a former Synergy 360 employee used parliamentary privilege to claim the firm planned to funnel money to former Liberal minister Stuart Robert.

Mr Robert, who is no longer a federal MP, and Synergy's co-owners have categorically denied the claim, saying no payments were made or sought.