In a landmark decision last year, Justice Stephen Rothman found 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 Nationwide News, Fairfax Media Publications and Australian News Channel.

The NSW Supreme Court lawsuit relates to 10 comments made about him by members of the public on Facebook posts on the pages of the Sydney Morning Herald, the Australian, the Centralian Advocate, Sky News Australia and the Bolt Report in 2016 and 2017.

Mr Voller is seeking damages, contending the comments, including a claim he attacked a Salvation Army officer detention visitor, leaving the man blind in one eye, carry false and defamatory imputations

The NSW Court of Appeal on Monday dismissed the challenge to the preliminary issue decided by Justice Rothman.

He did not err in concluding the companies were publishers of third party posts on their Facebook pages, said Justice John Basten sitting with Justice Anthony Meagher and Acting Justice Carolyn Simpson.

The companies later issued a statement saying they are considering whether to seek special leave to appeal to the High Court over the decision.

They had argued that to be publishers, they must have been instrumental to, or a participant in, the communication, whereas they had administered a public Facebook page on which third parties published material.

"Perhaps with a degree of hyperbole, they submitted that they were more closely equivalent to the supplier of paper to a newspaper owner or the supplier of a computer to an author," Justice Basten said.

But the court found someone who "participates in and is instrumental in bringing about publication of defamatory matter" may be liable for defamation even if others have taken part in the publication in different degrees.

"The applicants subscribe for a specific page and encourage and facilitate the making of comments by third parties which when posted on the page are made available to Facebook users generally."

They had sufficient control over the platform to be able to delete postings when they became aware that they were defamatory, the appeal court ruled.

Media companies Bauer Media, Dailymail.com Australia and Seven West Media had sought to become involved in the appeal proceedings but were not allowed to intervene in the case.

But the Broadcasting Services Act issue they wished to raise, which was not addressed by the three companies being sued, may be able to be argued at the full defamation hearing.

After Monday's ruling, Nationwide News, Fairfax Media and Australian News Channel said Australian defamation law was completely out of step with the realities of publishing in the digital age.

"The decision fails to acknowledge that it is Facebook that controls its platform, including that Facebook gives media companies no ability to turn off comments on their pages," they said.

"It is Facebook that must be held responsible for content posted by its users, not media companies."

Australian Associated Press