As stated above, 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 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.