For material to be defamatory it must be published to a person who is capable of understanding its defamatory significance. A letter is not published if it is in a language unknown to the reader. A letter read only by, and insulting only to, the person to whom it is addressed is not published. Defamatory material is published when it is communicated to someone other than the person it defames.
The publication must be intended to be, or the natural and probable consequence of, the initial actions of the person responsible for the defamation. If a thief steals a defamatory letter from a briefcase and reads it, or if a sealed envelope marked personal and confidential is opened and read by the victim's spouse, there is no publication. On the other hand if a person takes no reasonable precautions to avoid being overheard but instead talks loudly so others can hear the defamatory words this will amount to publication.
As publication involves no more than being responsible for communicating a statement, more than one person can be liable for the publication of defamatory material. An author, a printer and a publisher all publish a libel, and so does a distributor who knows that the material being distributed contains something that is defamatory. Every separate printed copy that is distributed is a separate publication.
It does not matter whether the publisher of the defamatory material intended to damage or injure the reputation of the person defamed, or acted with reasonable care [see Lee v Wilson (1934) 51 CLR 276;  HCA 60].
In relation to information available on the internet, it is generally held that it is the owner of the website that is deemed to be the publisher of the defamatory material [see Al Muderis v Duncan (No 3)  NSWSC 726].
The law in this area is developing, particularly in relation to the internet. For example, recent cases suggest that search engines such as Google and Yahoo! could be classified as publishers and be sued for defamation, however it is likely that, in most cases they would have a defence of an innocent disseminator [see Trukulja v Yahoo! Inc LLC & Anor  VSC 88].
However, recent cases have held that an operator of a search engine can be considered a secondary publisher of defamatory material, even though the operator may not have known that the material was defamatory in nature.
The recent cases of Duffy v Google Inc (2015) 125 SASR 437 and subsequent appeal in Google Inc v Duffy (2017) 129 SASR 304 identified that the defence of innocent dissemination should be modified to consider new technology. Accordingly, the majority of the Full Court of the South Australian Supreme Court dismissed Google’s appeal, and held that an internet publisher may be liable as the publisher of defamatory material if it fails to block the internet material after it has been made aware of the defamatory nature of the material [see Google Inc v Duffy (2017) 129 SASR 304 at 184-185]. The Full Court agreed that Google was a secondary publisher of the defamatory material, which was produced from other publications through Google’s search engine. This was held to be the case, even though the article was also otherwise available on the internet. In the original application, the plaintiff was awarded $100,000 in damages. The matter was upheld on appeal by the Full Court of the South Australian Supreme Court.
In Trkulja v Google LLC  HCA 25 the plaintiff claimed that Google had defamed him by publishing auto-predicted search results that linked him to organised crime and underworld figures. The High Court of Australia agreed and ruled that the plaintiff could pursue a defamation action against Google on the basis that the content of the auto-predicted searches could be defamatory.
In the case of Voller v Nationwide News Pty Ltd; Voller v Fairfax Media Publications Pty Ltd; Voller v Australian News Channel Pty Ltd  NSWSC 766 (Voller), the defendants (three different media organisations) maintained a public Facebook page, inviting contributions from members of the public (who also have Facebook accounts). The plaintiff alleged that some posts made on the public Facebook page by users were defamatory, and that the defendants should be held responsible for publishing the defamatory posts on its Facebook page. The court agreed with the plaintiff. The ruling in Voller held that social media platforms and media companies may now be considered a ‘publisher’ of comments made by third parties on their public pages, and they can be held liable for publication of defamatory material where those comments are defamatory.
The law of defamation and the internet is an expanding are in the law and caution should be taken when posting comments on social media platforms. The case law and decisions also demonstrate the development of potential liability in defamation of publishers, including social media platforms and internet search engines, who host and publish defamatory content posted by users or third parties.