For material to be defamatory it must be published (made available) to a third 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 by someone other than the person it defames, to another such person. This is known as bilateral publication.
In the High Court case of Dow Jones & Company Inc v Gutnick (2002) 20 CLR 575 at  the Court found:
Harm to reputation is done when a defamatory publication is comprehended by the reader, the listener, or the observer. Until then, no harm is done by it. This being so it would be wrong to treat publication as if it were a unilateral act on the part of the publisher alone. It is not. It is a bilateral act —in which the publisher makes it available and a third party has it available for his or her comprehension.
The publication must be the intended, or the natural and probable, consequence of the 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 by that person.
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 may all publish defamatory matter, and so may a distributor who knows that the material being distributed contains something that is defamatory.
This creates an important distinction between a primary (or main) publisher and a secondary (or subordinate) publisher. A primary publisher (such as an author, print or broadcast media publisher) can be held responsible for publication of defamatory material. In the case of a secondary publisher (such as a newsagent or bookseller) they may rely upon the defence of innocent dissemination [see below in Defences] if they can prove they “‘neither knew, nor ought reasonably to have known, that the matter was defamatory” and that the lack of knowledge was not due to any negligence on the part of the publisher [s 30] A primary publisher is irrefutably presumed to have the knowledge and be responsible for the defamatory material they have published. Therefore, the defence is not available to them.
For the purposes of commencing proceedings, permission of the Court may be required for multiple proceedings in relation to publication of the same defamatory matter [s 21]. A person may not bring further defamation proceedings for damages against a previous defendant or an associate of a previous defendant (an employee, contractor, associated entity, or employee or contractor of the associated entity) in relation to the same or any other publication of the same or like matter, except with permission of the Court.
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, the law in this area is still developing. It is generally held that the owner of the website is deemed to be the publisher of the defamatory material (in addition, where appropriate, to its author) [see Al Muderis v Duncan (No 3)  NSWSC 726].Recent cases have found that search engines such as Google and Yahoo! are secondary publishers and can be sued for defamation, although it is likely that, in most cases they would have the defence of innocent dissemination available [see Trkulja v Yahoo! Inc LLC & Anor  VSC 88 and Innocent Dissemination below.]
The recent cases of Duffy v Google Inc (2015) 125 SASR 437 and subsequent appeal in Google Inc v Duffy (2017) 129 SASR 304 dealt with the issue of innocent dissemination for a search engine. 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 remove 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 (unanimously) 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 decision, the plaintiff was awarded $100,000 in damages. This was upheld on appeal by the Full 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; Fairfax Media Publications Pty Ltd & Ors v Voller  NSWCA 102 (Voller), the three defendant media publishers each 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. Rothman J at first instance (on an interlocutory strike out application by the defendants) agreed with the plaintiff, and this was upheld on appeal. The ruling in Voller held that social media platforms and media companies can now be considered ‘publishers’ 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. Voller was appealed to the High Court, to determine the preliminary issue of whether the media publishers were in fact ‘publishers’ of comments made by third parties on their public Facebook pages. In September 2021 the HCA upheld the earlier NSW decisions, by finding that the publication element of the cause of action of defamation had been made out. The defamation action is now expected to return to the NSW Supreme Court for further pleadings and process to trial.
The law of defamation and the internet is an expanding area 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.