In the landmark ruling, Justice Stephen Rothman on Monday answered yes to the preliminary question of whether Mr Voller had established the companies were the "publishers" of each of the comments by third-party users that are alleged to be defamatory.

Mr Voller, whose mistreatment in the Don Dale Youth Detention Centre sparked a 2016 royal commission, is suing the Sydney Morning Herald, The Australian, The Centralian Advocate, Sky News Australia and The Bolt Report.

The NSW Supreme Court lawsuit relates to comments made about him by members of the public on 10 Facebook posts, published in 2016 and 2017, which he says carry false and defamatory imputations.

They include a claim he attacked a Salvation Army officer who visited him in detention, leaving the man blind in one eye, and also that he's a rapist.

Justice Rothman noted it was not alleged the companies were negligent in allowing the comments to remain on the public Facebook page, or reckless in failing to delete them once they were aware of defamatory material on their page.

In finding they were the publishers, the judge said their use of a public Facebook page was about their own commercial interests.

The primary purpose is to "optimise readership of the newspaper (whether hard copy or digital) or broadcast and to optimise advertising revenue," he said.

He acknowledged it is the third-party user that places the comment on the page.

But he said the evidence showed each company had the means effectively to delay the publication of the comments and to monitor whether any were defamatory, before releasing them to the general readership.

News Corp Australia later said the ruling showed Australia's defamation laws were out of step with other English-speaking democracies and highlighted the need for change.

"It defies belief that media organisations are held responsible for comments made by other people on social media pages," it said in a statement.

"It is ridiculous that the media company is held responsible while Facebook which gives us no ability to turn off comments on its platform bears no responsibility at all."

Justice Rothman said each defendant was not merely a conduit of the comment.

"It provided the forum for its publication and encouraged, for its own commercial purposes, the publication of comments," he said.

They were aware comments on their initial posting were likely, and more probably than not, would include defamatory material.

The judge was satisfied each company was a "first or primary publisher", meaning the defence of innocent dissemination would not arise.

Even if they had been found to be "subordinate or secondary", the judge said they were still the publisher and there seemed to be no public policy reason why liability should not be sheeted home to them.

"A defendant cannot escape the likely consequences of its action by turning a blind eye to it," he said.

Noting the legal issue of a public Facebook page appeared not to have been dealt with in previous cases, Justice Rothman acknowledged his decision may be appealed.

News Corp said it was considering an appeal.

Australian Associated Press