In Castlemaine Tooheys Ltd v The State of South Australia [1986] HCA 58; (1986) 161 CLR 148, 153, Mason ACJ summarised the principles governing the grant or refusal of an interlocutory injunction as follows:
In order to secure such an injunction the plaintiff must show (1) that there is a serious question to be tried or that the plaintiff has made out a prima facie case, in the sense that if the evidence remains as it is there is a probability that at the trial of the action the plaintiff will be held entitled to relief; (2) that he will suffer irreparable injury for which damages will not be an adequate compensation unless an injunction is granted; and (3) that the balance of convenience favours the granting of an injunction.
That summary was adopted by Gleeson CJ in Australian Broadcasting Corporation v Lenah Game Meats Pty Ltd [2001] HCA 63; (2001) 208 CLR 199, 217. These principles have been routinely applied in this and other courts in Australia.
These principles were further explained by Gummow and Hayne JJ in Australian Broadcasting Corporation v O'Neill [2006] HCA 46; (2006) 227 CLR 57, [65] - [71] (Gleeson CJ and Crennan J agreeing). Their Honours stated that the relevant principles are those stated in Beecham Group Ltd v Bristol Laboratories Pty Ltd [1968] HCA 1; (1968) 118 CLR 618, where the two main inquiries were said to be whether the plaintiff had made out a prima facie case and whether the balance of convenience favours the grant of the injunction. The phrase 'prima facie case' does not mean that the plaintiff must show that it is more probable than not that at trial the plaintiff will succeed. It is sufficient that the plaintiff show a sufficient likelihood of success to justify, in the circumstances, the preservation of the status quo pending the trial. How strong the probability needs to be depends upon the nature of the rights the plaintiff asserts and the practical consequences likely to flow from the orders the plaintiff seeks: [65], [71].
The apparent statement by Lord Diplock in American Cyanamid Co v Ethicon Ltd [1975] UKHL 1; [1975] AC 396, 407 that, provided the court is satisfied that the plaintiff's claim is not frivolous or vexatious, there will be a serious question to be tried, is not to be followed. The governing consideration is that the requisite strength of the probability of ultimate success depends upon the nature of the rights asserted and the practical consequences likely to flow from the interlocutory orders sought. These principles make it clear that the various considerations identified by Mason ACJ in Castlemaine Tooheys are to be considered together.
As the apparent strength of the applicant's case diminishes, the balance of convenience moves against the making of an order: Glenwood Management Group Pty Ltd v Mayo [1991] VicRp 45; [1991] 2 VR 49, 54-55; Todd v Novotny [2001] WASC 171. The grant of an injunction involves balancing the injustice which might be suffered by the defendant if the injunction is granted and the plaintiff later fails at trial, against the injustice which might be suffered by the plaintiff if the injunction is not granted and the plaintiff later succeeds at trial: Films Rover International Ltd v Cannon Film Sales Ltd [1987] 1 WLR 670; Madaffari v Labenai Nominees Pty Ltd [2002] WASC 67 [14].