Media companies being sued by ex-NT youth detainee Dylan Voller have lost their bid to overturn a ruling they are liable for postings made on their Facebook pages.

In a landmark decision in June last year, Justice Stephen Rothman agreed 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.

The NSW Court of Appeal on Monday dismissed the challenge to Justice Rothman’s ruling.

The three judges said a person who participates in or is instrumental in bringing about the publication of defamatory matter is potentially liable for that act, notwithstanding that others may have participated in that publication to different degrees.

In this case, the sites maintained Facebook pages and encouraged and facilitated the making of comments by third parties which were made visible to Facebook users generally.

They were therefore publishers of the comments.

-AAP