Officers from intelligence agency ASIO entered the ABC's offices in Canberra and Brisbane early on Thursday to secure classified and top secret cabinet documents recently obtained by the public broadcaster under extraordinary circumstances.

At about 1am AEDT, ASIO agents delivered safes to locations in Parliament House and at South Bank in Brisbane to store the thousands of pages of sensitive documents, accessed by the ABC after they were discovered in locked government filing cabinets at a used furniture store.

The ABC still has access to the files and its lawyers are negotiating with the Department of Prime Minister and Cabinet (PM&C) regarding the future of the information.

PM&C launched an urgent investigation on Wednesday into how the papers - which are supposed to be kept secret for 20 years - ended up being sold to a member of the public.