Hundreds of top-secret and highly classified documents from the Australian government have been found in a second-hand filing cabinet, according to local media.

The trove of files was revealed by broadcaster ABC, which described the so-called 'Cabinet Files' as "one of the biggest breaches of cabinet security in Australian history".

According to the national broadcaster, the filing cabinet ended up in a second-hand shop in the capital Canberra where ex-government furniture is often put up for sale.

It adds: "The deals can be even cheaper when the items in question are two heavy filing cabinets to which no-one can find the keys."

The locks were eventually opened with a drill, revealing the documents inside.

They cover five separate governments, and many of the files would legally have remained secret for 20 years.

Nearly all the files are described as classified, with some labelled top secret.

ABC has published some documents, but says it has withheld others for national security reasons or to protect the privacy of public servants.

Among the revelations are that almost 200 sensitive documents were left in the office of a former senior minister after the Australian Labor party lost power after the 2013 election.

The reports also say that a group of government officials under former Prime Minister Tony Abbott's government "considered banning anyone under 30 from accessing income support".

AFP reports that the Australian Department of the Prime Minister and Cabinet has "initiated an urgent investigation" following the discovery of the files.