Filing cabinets containing thousands of classified documents from the Australian government ended up being sold at a secondhand shop, prompting government officials Wednesday to launch an investigation into how the highly sensitive documents were disposed of.

The cache of documents was obtained by the Australian Broadcasting Corporation, which reported the two cabinets were sold by a Canberra furniture shop at a discount price because they were locked and no one could find keys.

The ABC described the episode as an "extraordinary breach of national security," and that nearly all the documents are classified with some documents being marked "top secret," "sensitive," "Australian eyes only," and "cabinet-in-confidence."

The state-owned broadcaster did not say when the documents, known as the Cabinet Files, were found or whom the buyer was who removed the locks with a drill -- but in recent weeks the outlet has reported stories on embarrassing moments during the tenures of former Prime Ministers Kevin Rudd and Tony Abbott.

The documents contain information detailing Australia's intelligence priorities, counterterrorism strategies, missile upgrades and profiles of terror suspects that spans more than a decade and four prime ministers, the most recent being Abbott. Abbott was replaced in 2015 by current Prime Minister Malcolm Turnbull.

When questioned about the ABC's reporting of Cabinet documents on Tuesday, Turnbull told reporters: "I think they've come across someone's bottom draw in Canberra," Sky News reported.

Some of the files reveal former finance minister Penny Wong left nearly 200 top-secret papers in her old office when her government was voted out in 2013. The documents included Middle East defense plans, national security briefs and updates on the war in Afghanistan.

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The files also revealed that former Prime Minister John Howard's National Security Committee at one point gave serious consideration to removing the right to remain silent to terrorism suspects when being questioned by police.

"I would also like NSC to consider whether amendments should be made to a suspect's right to remain silent to allow a court to draw adverse inferences in a terrorism trial where an accused relies on evidence which he or she failed to mention when questioned by police," Ruddock wrote in his NSC submission obtained by the ABC.

The proposal was then supported by the Australian Federal Police and Australian Security Intelligence Organization, but later rejected by the majority of the committee.

Another document reveals that AFP had lost almost 400 national security files over five years ending 2013. While the content of the missing security files was not revealed, "troop deployments in Afghanistan and Iraq, counterterrorism operations, foreign relations and Australia's border protection were among the top-secret and sensitive issues decided in the five-year period," according to the ABC.

Australian Cabinet documents are usually kept secret for 20 years before they are made public in a heavily redacted form. The sale of the ex-government furniture in the country's capital was not limited to Australians, meaning anyone could make a purchase and could have handed the contents to a foreign agent or government.

The ABC said it chose to publish some of the files because "national security and the inner workings of our government affect the lives of all Australians."

The broadcaster said it had not chosen to report on some of the documents on national security grounds, but said the leak shows the "casual attitude of some of those charged with keeping the documents safe."

Rory Medcalf, head of the Australian National University's National Security College, told The Associated Press the matter was "very weird and embarrassing." He added that while the U.S. and other Aussie allies should be concerned, the leak is not as damaging as prior ones.

"This is not catastrophically damaging for national security in the sense that that something like the Snowden revelations must have been," Medcalf told the AP, referring to the former U.S. National Security Agency contractor Edward Snowden, who disclosed classified information in 2013.

The Associated Press contributed to this report.