Fairfax Media has previously reported on one aspect of the allegations, which involved defence staffers colluding with contracting companies to design handsomely-paid jobs with requirements tailored to their own experience.

But a confidential minute written by the head of the department's internal audit division described how the allegations also related to "systemic" breaches of procurement practice over several years.

Defence has not said which sections of the agency were investigated, however it is suggested in the minute that the department's head of maritime systems be informed of the findings from an internal investigation.

Explicit mention was also made of the department's official policy on international travel.

Details of the investigation are revealed in heavily-redacted internal documents released under freedom of information laws.

The investigation, which finished in February this year, was prompted by an anonymous disclosure about fraud and corruption made to the department in April 2016.

Dr Tom Clarke, the head of defence's audit and fraud control division, commented on the "systemic" nature of the claims in a minute to the assistant secretary of fraud control.

"Due to the systemic nature of the allegations, Audit Branch was requested to conduct a review to substantiate the allegations in relation to procurement practices over several years," Dr Clarke wrote.

The investigation focused on the department's compliance with legislation such as the Financial Management and Accountability Act and the Public Governance, Performance and Accountability Act.

According to the files, the investigation into the allegations ran for 10 months between April 2016 and February this year.

The Department of Defence refused to release details of the investigation, saying such information could compromise future audits.

However, a departmental spokesman has previously commented on some aspects of the audit's outcome.

"An initial assessment of the matter determined that there was insufficient evidence to substantiate the allegations relating to fraudulent and corrupt conduct," the spokesman said in October.

"The internal audit activity did find instances of non-compliance with procurement policy requirements, a lack of transparency in decision making and poor records management."

The defence spokesman said the allegations had not been referred to any external agency for further investigation.

"While it was determined there was no evidence to support the allegations of fraudulent and corrupt conduct, there were sufficient concerns held to justify an internal audit to determine whether the procurement activities had followed procurement rules and policy."

The internal investigation did not make any "adverse findings" against any individual employee, the spokesman said.

However, none of the individuals named by the whistleblower were currently employed on defence contracts.

There was also "insufficient evidence" regarding fraudulent and corrupt conduct on behalf of the contracting companies in question, although one company was no longer engaged on defence contracts.

In 2015-16 the Department of Defence reported the largest yearly contract spend of any agency within the public service.

The department dialled-up a bill of $30.5 billion on contracts, amounting to 54 per cent of total government contractor spending.

The department has recently been taking steps to avoid conflicts of interest that might arise when a public servant moves into the private sector.

Dr Clarke has previously commented on the need to manage potential conflicts of interest within the Defence Force.

"Current and former officials of defence are custodians of our special reputation in the Australian community," he told a previous issue of the Navy News.

"Through our actions we can build, or erode, that reputation.

"When it comes to post-separation employment, we must act and be seen to act with the highest standards of probity."