Australia has suffered a “momentous” security breach involving the publication of cabinet discussions spanning more than a decade after two locked filing cabinets were bought from a second-hand shop and prised open, revealing thousands of secret documents.

According to ABC News, which obtained the documents, the filing cabinets were sold at a discount by a furniture shop in Canberra because they were locked and no-one could find the keys.

An anonymous person bought the cabinets and several months later opened them with a drill, unveiling a trove of documents marked “top secret” or “AUSTEO” (meaning for Australian-eyes only).

The discovery prompted the head of Prime Minister Malcolm Turnbull’s department to order an urgent investigation and has already resulted in embarrassing revelations about the governments of former leaders Kevin Rudd and Tony Abbott.