ABC chair Ita Buttrose has "grave concern" about this week's Australian Federal Police (AFP) raid at the national broadcaster, which she says was "clearly designed to intimidate".
AFP officers executed a search warrant at the ABC's Ultimo headquarters on Wednesday, over a series of 2017 stories known as the Afghan Files.
In a statement, Ms Buttrose said she had a "frank conversation" with Federal Communications Minister Paul Fletcher on Thursday, and that the raid was "clearly designed to intimidate".
The AFP also raided the Canberra home of a News Corp journalist on Tuesday - however that incident was not related to the ABC search warrant.
"It is impossible to ignore the seismic nature of this week's events: Raids on two separate media outfits on consecutive days is a blunt signal of adverse consequences for news organisations who make life uncomfortable for policy makers and regulators by shining lights in dark corners and holding the powerful to account," Ms Buttrose said.
"I also asked for assurances that the ABC not be subject to future raids of this sort.
"Mr Fletcher declined to provide such assurances, while noting the 'substantial concern' registered by the corporation."
The Afghan Files stories, by ABC investigative journalists Dan Oakes and Sam Clark, revealed allegations of unlawful killings and misconduct by Australian special forces in Afghanistan and were based off hundreds of pages of secret Defence documents leaked to the ABC.
"As ABC chair, I will fight any attempts to muzzle the national broadcaster or interfere with its obligations to the Australian public," Ms Buttrose said.
"Independence is not exercised by degrees. It is absolute."
Ms Buttrose, who is only the second woman to lead the ABC board, said an "untrammelled" media which upholds the community's right to information has driven her journalistic career for almost five decades.
"In my view, legitimate journalistic endeavours that expose flawed decision-making or matters that policy makers and public servants would simply prefer were secret, should not automatically and conveniently be classed as issues of national security."
Ms Buttrose called the search warrant, which named Oakes, Clark and the ABC's Director of News Gaven Morris, as "sweeping".
The raid sparked a global conversation about the public's right to know, with many expressing concern over threats to press freedom and democracy.
The New York Times published an article addressing the raids with the headline Australia May Well Be the World's Most Secretive Democracy.
Prime Minister Scott Morrison defended the raid and told journalists the upholding of Australian laws "never troubles" him.
After the raid, that lasted more than eight hours, the AFP officers took with them two USB drives containing a small number of electronic files, which were sealed in plastic bags pending a review by ABC lawyers.
The ABC has two weeks to challenge the terms of the warrant as well as claim privilege over the contents of the documents before the AFP can unseal that bag and, potentially, use the files as evidence.
Managing director David Anderson said ABC journalists' sources were safe.