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 defendant 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 defendant towards the plaintiff 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.