The documents were contained in two cabinets that were sold locked and without matching keys at an ex-government furniture shop in Canberra.
The broadcaster said the files could easily have fallen into the wrong hands as the sale was not limited to Australians and the documents could have been passed on to a foreign agent or government.
They were eventually bought and left unopened until the purchaser used a drill to break off the locks.
Under Australian law, the secret cabinet files would have become public only after 20 years had passed, in order for senior ministers to feel they can speak openly and freely in the sanctity of the cabinet room. Some were marked top secret and others code-word-protected - the highest classification of sensitivity, according to reports.
"This has been achieved without compromising the ABC's priority of protecting the integrity of its source and its reporting, while acknowledging the Commonwealth's national security interests", the joint statement said.
The documents belonged to the office of former Minister Penny Wong, leader of the government in the Senate in 2013, and as a member of the National Security Committee, she had access to this confidential information.
"Obviously some absolutely elementary mistake has been made, presumably by a relatively junior or mid-ranking departmental officer", former prime minister Tony Abbott said, according to the ABC.
ABC reported that ASIO seized hundreds of the documents on Thursday, but it was not immediately clear whether all were retrieved.
The move comes as former Prime Minister Kevin Rudd launched legal action against the ABC after it published details from a secret cabinet document related to his Labor government's home insulation scheme.
Terry Moran, who was PMC secretary from 2008 to 2011, told ABC's 7.30 the discovery was a "great surprise" and whoever was responsible for disposing of the cabinets "must be found and sacked".
In response to The Cabinet Files, Rudd told the ABC any assertion he was warned about safety risks was untrue.
John Howard's National Security Committee (NSC) nearly removed the right to remain silent.
"The thousands of pages reveal the inner workings of five separate governments and span almost a decade".
"Journalism like this relies on courageous confidential sources and we'll protect their privacy at all costs".
Hours earlier, ASIO had provided document safes to the ABC offices to safeguard the files while lawyers negotiated over what to do with the papers that include Cabinet secrets spanning nearly a decade.