As reviews of businesses take off on social media, the District Court of Western Australia was asked to consider whether a series of posts on a Facebook page for UK emigrants, "Poms in Perth", had defamed a local accountant.

Stephanie Massey posted an unflattering review of Joondalup accountant Barry McEloney in August last year, telling users "whatever you do, DO NOT use" him to advise on tax returns.

"That clown wanted $550 ... antagonised my boyfriend and hung up on me, interrupting me several times prior. DEFO not worth $550 in my eyes," she said in a second post on the members-only page.

The accountant, who was recommended to Ms Massey's boyfriend by a friend, sued for defamation over the posts. In her defence, Ms Massey pleaded that the statements were true or alternatively were fair comment.

Judge Anette Schoombee found for Ms Massey and threw out the case. She said the claims that Mr McEloney was "rude and unprofessional, displayed poor interpersonal skills ...were, at least substantially, true".

She said the claim that Mr McEloney overcharged for his services had not been shown to be true but it was defensible as an honest opinion, as were suggestions that he should not be retained.

Defamation law expert David Rolph, an associate professor at the University of Sydney law school, said the dispute was "a really rare case where a defence of honest opinion succeeds".

The defence was aimed at protecting freedom of speech but was "highly technical" in its application, he said.

The defence requires the opinion to be related to a matter of public interest and based on "proper material", which usually means something that is substantially true.

Judge Schoombee said readers would appreciate comments made on a Facebook page were not the same as those on the front page of a newspaper, and users "often write in half sentences, with abbreviations, keep their comments brief and use colloquial language".

Comments made could be "a bit one-sided or exaggerated" but readers would approach reviews "with appropriate caution and a bit of scepticism".

"The ordinary reasonable reader would also realise that what one person might regard as 'overcharging' might be a reasonable fee to another to whom a meticulous service or a reliable service provider was important," she said.

But Judge Schoombee added that "this does not mean that a member of a Facebook page has carte blanche to defame service providers or other persons or that all statements made necessarily qualify as opinions".

In a separate case in Victoria, the Supreme Court considered whether an administrator of a Facebook page could be liable for comments posted by another user. Justice John Dixon said that the administrator could be liable as a secondary publisher as they had the power to remove the comments. Associate Professor Rolph said the case applied long-established principles.

"Virtual walls are just like physical walls. If you are responsible for a piece of property and someone puts something defamatory on it, someone tells you about it and you have the power to take it down and don't do it, the law has always been that you become responsible for it yourself," he said.