Authorities are still no closer to explaining how a classified notebook belonging to a top Defence official found its way into second-hand furniture sold at a Canberra tip.
The Defence Department is understood to have since launched an internal policy review as it investigates this latest national security breach.
On Thursday, a Jerrabomberra man handed Fairfax Media thesmall pink notebook, which references Australian counter-terrorism operations.He claims to have found the notes, along with three security passes belonging to the senior official, taped to the back of an unlocked drawer in an old cabinet he picked up in December.
Defence locked down and then removed the notebook from The Canberra Times newsroom on Friday. Associate secretary of Defence Rebecca Skinner said it could "substantially harm" national security if the information fell into the wrong hands.
The man, who did not wish to be named, said he made the discovery just hours after it was revealed thousands of sensitive documents, known as 'The Cabinet Files', were accidentally sold at a second-hand government furniture store in Fyshwick and obtained by the ABC.
When the news broke, it prompted him to look inside his own cabinet.
"It was taped up in a plastic bag right at the back," he said.
"I was surprised, but not that surprised really given what's been on the news. It's pretty alarming."
A Defence spokesman confirmed none of the items were reported stolen, but initial inquiries suggested they were accidentally thrown out with a piece of personal furniture.
The man claims he bought the cabinet for just $20 from thrift store The Green Shed at the Mugga Way tip, but staff say they can't remember it.
"Then again we do sell thousands of things in a week," Green Shed co-owner Sandie Parkes said.
"But we don't tend to get much government furniture. We've never had anything like this before, as far as we know."
It is understood Defence secretary Greg Moriarty has since called for a wider, systemic look at the department's security policies, in an investigation expected to take a number of weeks.
The department requested Fairfax withhold the identity of the senior official involved in the breach, as "publication of such information may give rise to a foreign intelligence threat".
The official wrote the notes while working with the Australian Signals Directorate in 2016.
A Defence spokesman said he could not comment further on the investigation at this time.
"Defence would like to take this opportunity to thank The Canberra Times for supporting the investigation and co-operating with security officers to ensure the secure return of all classified documents to the Commonwealth," he said.
Handwritten notebooks containing classified information are subject to the same security protocols as printed or digital documents, but the onus is on government officials to keep them safe.
"Defence security policy requires all information produced within Defence or by Defence personnel to be classified and marked by the originator," the spokesman said.
Officials are also required to report lost or stolen passes, but a lost pass was considered a minor security incident.
On Friday, the Department of Prime Minister and Cabinet admitted it had lost the Cabinet Files, which were taken back from the ABC by ASIO last week.