Australian Federal Police officers are raiding the ABC's Sydney offices over a series of stories about Australia's elite special forces in Afghanistan published in 2017.
The raid comes a day after the AFP spent seven and a half hours searching the home of NewsCorp political journalist Annika Smethurst.
The ABC's Head of Investigative Journalism John Lyons said the AFP had issued a warrant naming three journalists, including news director Gaven Morris.
In a series of tweets, Mr Lyons said officers had told him they were interested in "certain things" relating to the stories known as 'The Afghan Files'.
Based on defence documents obtained by the broadcaster, the report exposed a "growing unease" within the Defence department about Australia's special forces.
In a statement, the AFP said it executed a search warrant on the Ultimo premises in relation to "allegations of publishing classified material".
The statement revealed the matter was referred to police the day the story was published by the Chief of the Defence Force and the then Acting Secretary for Defence.
On Tuesday, AFP officers searched Annika Smethurst's Canberra's home over a 2018 report detailing a proposal to spy on Australians that included images of letters between the heads of the home Affairs and Defence departments discussing potential new powers.
The AFP said the two searches are unrelated.
Morrison shrugs off raid
The police action targeting reporters has alarmed many journalists, who have raised concerns about freedom of the press.
Prime Minister Scott Morrison, who's on his first official visit to the UK as PM, indicated he was comfortable with what happened.
When pressed on whether it "bothered" him to see a reporter's home being raided, the prime minister appeared unconcerned.
"It never troubles me that our laws are being upheld," Mr Morrison smiled.
Labor has asked for a briefing on what occurred, with new leader Anthony Albanese describing it as "extraordinary".
Centre Alliance Senator Rex Patrick believes the intimidation of a journalist is a "worrying development".
He plans to follow up the matter in Senate estimates hearings later in the year.
"There are serious questions about the timing of the raid, why the raid occurred only at her home, and indeed what steps had been taken prior to the raid being conducted," he told ABC Radio on Wednesday.
News Corp labelled the raid "outrageous and heavy handed".
"This raid demonstrates a dangerous act of intimidation towards those committed to telling uncomfortable truths," a spokesperson said.
"What's gone on this morning sends clear and dangerous signals to journalists and newsrooms across Australia. This will chill public interest reporting."
