Inside, they discovered files that are almost all classified, with some designated "top secret" and others marked "AUSTEO", meaning they were meant to be seen by Australian eyes only.
The documents were then passed onto the Australian Broadcasting Corporation (ABC) that ran a string of stories, embarrassing former prime ministers and several lawmakers who still occupy prominent positions inside Australia's center-right government.
The papers left in the office - but not included with the sold filing cabinets - included Middle East defence plans, national security briefs, Afghan war updates, intelligence on Australia's neighbours and details of counterterrorism operations.
ABC chose to publish some of the files, containing information regarding Australia's counterterrorism strategies, missile upgrades and terror suspect profiles, because "national security and the inner workings of our governments affect the lives of all Australians".
All discussions inside Cabinet are considered confidential and documents on Australian Cabinet decisions are supposed to remain confidential for at least to 20 years. The broadcaster opted not to disclose all files for national security reasons and to protect the privacy of public servants.
Later, the buyer broke the locks and found thousands of official documents that were eventually handed over to the ABC, which named the treasure as the "Cabinet Files".
Tony Abbott's government meant to ban anyone under 30 from accessing income support.
The department launched an urgent investigation on Wednesday, after it was revealed the trove of documents had been discovered in two locked filing cabinets offloaded to a second-hand furniture depot in Canberra.
Despite the highly classified nature of the secret cabinet files, the ex-government furniture sale was not limited to Australians.
On Wednesday, the Department of the Prime Minister and Cabinet (PMC) announced an urgent investigation.
The documents span the administrations of four prime ministers, the last being Tony Abbott, who was succeeded by current Prime Minister Malcolm Trumbull. The documents reported this week were said to delve into the workings of five previous governments over the last decade.
Former prime minister Kevin Rudd has launched legal action against the ABC after it published details of a secret cabinet document related to his Labor government's home insulation scheme.
The cabinets full of state secrets were sold at an op shop. The documents came from the cabinet's national security committee, which controls security, intelligence and the defense agenda in the country.