By Mike Cherney

SYDNEY--Newspapers and television stations that post their own articles on Facebook Inc.'s platform are liable for other Facebook users' defamatory comments on those posts, an Australian court ruled Monday, presenting a fresh dilemma for traditional publishers in the social-media age.

Media companies encourage and facilitate comments that can be seen by other Facebook users, said the highest court in the Australian state of New South Wales, which includes Sydney. That means, it ruled, that the companies should be considered publishers of the comments, responsible for their content.

The media companies that were defendants in the original lawsuit, which include News Corp Australia as well as the publisher of the Sydney Morning Herald, said they are considering a further appeal to the country's highest court.

"Today's decision means the media cannot share any story via Facebook without fear of being sued for comments which they did not publish and have no control over," they said. "It also creates the extraordinary situation where every public Facebook page--whether it be held by politicians, businesses or courts--is now liable for third-party comments on those pages."

News Corp Australia is a subsidiary of News Corp, which also owns Dow Jones & Co., publisher of The Wall Street Journal.

The decision could also threaten social-media platforms like Facebook, which count on news articles for traffic and ad revenue. In April, Australian authorities said they would require Facebook and Alphabet Inc.'s Google to pay local media organizations for their content, amid a broader debate over whether the tech companies are unfairly benefiting from news articles on their platforms.

Australia has also moved to hold social-media companies themselves responsible for what users post. After a live stream of a shooting spree at New Zealand mosques last year was posted on Facebook, Australia passed legislation that allows social-media platforms to be fined if they don't remove violent content quickly.

Monday's case was initially brought by Dylan Voller, who was detained in a juvenile detention center and became the subject of media attention. Articles about Mr. Voller that media outlets posted on Facebook drew comments from other Facebook users falsely accusing him of serious crimes, according to his lawyer, Peter O'Brien.

The court heard evidence that traditional media companies use Facebook to drive traffic to their own websites, Mr. O'Brien's statement said: "With this strong commercial imperative driving them, it really is a no-brainer that the media companies lent their assistance to the publication of third-party comments."

In the U.S., Section 230 of the Communications Decency Act broadly exempts social-media companies like Facebook from legal liability for what people post on the site. However, after Twitter Inc. last month applied a fact-checking notice to tweets by President Trump, the president signed an executive order that could curb some of those legal protections.

Michael Douglas, a defamation lawyer and senior lecturer at the University of Western Australia, said if Monday's ruling stands, traditional media companies could be forced to beef up monitoring of third-party comments on their social-media posts. Aside from an appeal, Mr. Douglas said he'd expect the companies to lobby state and federal governments to change defamation laws. A review of the laws is already under way.

"It's a big challenge to the business model of publishers, because it means there is a greater risk any time you create content which is in any way controversial," he said. "There is a risk that users will write something objectionable, which will open up the entity behind the account to being sued for defamation."

The media companies said Monday that the court's ruling failed to acknowledge that Facebook doesn't give media companies the ability to turn off comments. Facebook should be held responsible for content posted by users, they said.

Facebook pages generally offer some ability to moderate content, including hiding and deleting comments.

"We are aware of the court's decision today and we are reviewing it carefully," Facebook said.

Monday's ruling, which upheld a decision from last year, didn't determine whether the Facebook comments regarding Mr. Voller were in fact defamatory.

Write to Mike Cherney at mike.cherney@wsj.com