The Australian Security Intelligence Organisation has warned against exempting journalists from national security laws because hostile foreign spies will exploit them in their operations against the country.

The message about foreign intelligence using the "ideal cover" of journalism was aired in Parliament's inquiry into press freedom. During a public hearing on Wednesday, the Australian Federal Police declined to rule out pursuing charges against News Corp journalist Annika Smethurst over the leak of secret government documents.

The headquarters of the Australian Security Intelligence Organisation. Credit:Sitthixay Ditthavong

Michael Pezzullo, the secretary of the Department of Home Affairs, also indicated police are "closing in" on the the suspected leaker of the documents. Mr Pezzullo lashed out at the disclosure as "completely unacceptable" and said the person should go to jail.

ASIO, the security agency charged with counter-espionage in Australia, told the hearing in Canberra the profession of journalism could be used "nefariously" by foreign intelligence services.