The law already provided Mr Voller with a remedy in that he could sue the people who made the comments, James Hmelnitsky SC said in the NSW Supreme Court on Friday.

The barrister - who's acting for the Sydney Morning Herald, The Australian, The Centralian Advocate, Sky News Australia and The Bolt Report - was making final submissions in a landmark defamation case.

Mr Voller, whose mistreatment in the Don Dale Youth Detention Centre sparked a royal commission, is suing the companies over 10 Facebook posts published in 2016 and 2017 which he says carry false and defamatory imputations.

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

His barrister, Tom Molomby SC, has argued the media companies are liable for the defamatory comments left on their Facebook pages.

"The evidence is they had no systematic policy of monitoring in place," he said on Friday.

"They say they don't have the capacity because of the quantity of traffic and yet they must have known it was highly likely there would be defamatory comment."

But Mr Hmelnitsky referred to previous cases when it was found a defendant can be considered to be a publisher only after they are given notice of defamatory content.

"There was no complaint about these publications. There was no email, letter or phone call."

The very first notice given to the defendants was when they were served with the statements of claim, he said.

"The liability only extends from the time when you have been given notice," he said.

"There is no authority for the proposition that you can be held liable in these circumstances, without knowledge."

Mr Voller's reputation was protected by the law, Mr Hmelnitsky added.

"Every single one of these commenters can be sued."

Justice Stephen Rothman, who acknowledged that the losing party was likely to appeal, reserved his decision.

Australian Associated Press