Prime Minister Malcolm Turnbull’s department has launched an “urgent investigation” into how two filing cabinets containing classified and sensitive government documents were mistakenly sold at a second hand furniture shop in Canberra.

In an extraordinary lapse of security, the documents, which include federal cabinet papers that should not be made public for 20 years, were apparently bought by a member of the public and eventually passed on to the ABC.

A spokesperson for the Department of Prime Minister and Cabinet said: “The secretary of the department ... has initiated an urgent investigation into the circumstances around the disposal of two Commonwealth government filing cabinets that alledged contained classified material. Given that the investigation is underway, it is not appropriate for the department to comment further at this time."

The ABC has reported the filing cabinets were locked when the unidentified person bought them. The buyer eventually drilled open the locks and discovered thousands of pages of documents.

Nearly all the files were classified, some as “top secret” or for Australian eyes only, the ABC reported.