More than 800 files are being examined by the Australian Federal Police as it seeks to determine whether criminal activity was involved in the government's $33 million purchase of land near the Western Sydney airport.
Deputy Commissioner Ian McCartney said the agency was "at a very early juncture" in the investigation and so far hadn't identified any crime in the sale of the land, where the government spent 22 times what the NSW government paid for its portion of similar land.
The investigation was officially opened on July 27. The Auditor-General first called AFP Commissioner Reece Kershaw about the issue on July 10, followed by an official referral on July 13.
As Auditor-General Grant Hehir told a different Senate estimates on Monday night that it was the first time the audit office had referred a matter to the federal police in at least 20 years.
Commissioner Kershaw said he told the Department of Infrastructure in October that an investigation was already underway when the department's secretary, Simon Atkinson, attempted to make a referral to the federal police after receiving the audit report.
On Monday Mr Hehir said he referred the purchase of the land to the Australian Federal Police because there were too many gaps in information about how the decision had been made to pay the inflated amount, and it suggested the Commonwealth might have been defrauded.
The Infrastructure Department in 2018 purchased about 12 hectares of land near the Western Sydney Airport from the Leppington Pastoral Company for $33 million.
When the land was re-valued, it was found to be worth $3 million.
"We thought this was a significant movement in 12 months," Mr Hehir said.
The agency began a performance audit but soon "gaps in the evidence" emerged.
"There was a whole sequence of actions we couldn't explain," Mr Hehir said.