Extended secondary liability was the subject of R v Keenan  HCA 1 (2 February 2009). The High Court of Australia was here considering s 8 of the Criminal Code (Q):
"When 2 or more persons form a common intention to prosecute an unlawful purpose in conjunction with one another, and in the prosecution of such purpose an offence is committed of such a nature that its commission was a probable consequence of the prosecution of such purpose, each of them is deemed to have committed the offence."
This sort of liability has common law and statutory forms, and it is important to notice the aspects of the definition that differentiate it from others. Here the phrase "of such a nature" and the objective nature of the probability ("was a probable consequence", not the subjective formulation "was known to be a probable consequence") are significant. An example of a different formulation is s 66(2) Crimes Act 1961[NZ].
In Keenan an alleged common purpose of inflicting serious physical harm on the victim was followed by the use of a gun by one of the offenders to cause grievous bodily harm to the victim. Was Mr Keenan, the respondent in this appeal, correctly convicted of doing grievous bodily harm with intent to do that harm, pursuant to s 8, if use of a gun was not part of the common purpose?
For discussion of the common law, see Rahman  UKHL 45 (blogged here 3 July 2008). Kirby J (dissenting) preferred not to interpret s 8 in a way that would depart from the common law, which left to the jury the task of determining the boundary of secondary liability in the particular circumstances.
The other members of the Court held that it is not the way that the harm is caused that matters (the fact that it was by use of a gun) but rather it is the nature of the harm that was caused (grievous bodily harm). Then the questions are, what was the common purpose, and was the offence that occurred (in a generic sense, not the precise acts) a probable consequence of the prosecution of that purpose. As Hayne J pointed out (83), this gives effect to the phrase "of such a nature", whereas Kirby J's interpretation would focus on the way the offence was committed and ask whether that was a probable consequence of the prosecution of the common purpose. Kiefel J (with whom all the majority agreed) held (115) that the act involved in the commission of the offence is not part of the connection between the common purpose and the offence.
Kirby J supports his dissent with policy grounds (66), emphasising the importance of the jury's role as the setter of the boundaries of liability in accordance with community values.