Lord Birkett said in 1951 - 'It is the right of every man to comment freely, fairly and honestly on any matter of public interest.' This principle means it is not defamatory when words are an expression of opinion and not a statement of fact. It may be difficult to determine if words are statements of facts or expressions of opinion. Words must be construed in their context and in some circumstances words that would otherwise be statements of fact might be viewed as comments.
The opinion, however, must be fair and based upon facts which the respondent can identify and prove to be true. It must also be honestly held and not motivated by malice (some improper or dishonest motive). Personal ill will by the respondent towards the applicant is an example of malice. 'Public interest' is a wide concept involving what is a legitimate concern to the public. Instances include the conduct of people holding public office, the conduct of a political party, the conduct of a clergyman and artistic works such as plays and books. It extends as far as the criticism of a restaurant's food in a newspaper review.
The Defamation Act 2005 (SA) s 29 provides a defence of honest opinion where the expression of opinion is related to a matter of public interest and based on ‘proper material’ that is substantially true or based on privileged material.
Consumers leaving reviews online should be prepared to justify their opinion. In Cheng v Lok  SASC 14, the Supreme Court o South Australia awarded damages to a lawyer following a bad review of his services on Google. The plaintiff had never met the defendant, was never retained as the defendant’s lawyer and had never represented the defendant in any capacity. The decision in Cheng illustrates that negative Google reviews will be considered publications for the purposes of the Defamation Act 2005 (SA).
Caution should be exercised when expressing opinions and reviews online, even anonymously.
In Kabbabe v Google LLC  FCA 126 the court made interlocutory orders allowing the plaintiff to serve a court application on Google outside of the Australian jurisdiction for the purposes of seeking preliminary discovery, and to request that Google to reveal the identity of an anonymous online reviewer. The court held the view that this was reasonable, as the plaintiff had a prima facie case for defamation against the reviewer following a negative online review about the plaintiff’s business. The court held the view that it was not sufficient for Google to say that the review had been posted anonymously, as Google had the capacity to identify the person leaving the review.