"Where 2 or more persons form a common intention to prosecute any unlawful purpose, and to assist each other therein, each of them is a party to every offence committed by any one of them in the prosecution of the common purpose if the commission of that offence was known to be a probable consequence of the prosecution of the common purpose."
So says s 66(2) of the Crimes Act 1961 [NZ], defining extended secondary participation in offending. The contentious phrase has been "a probable consequence", giving rise to arguments about whether a common intention to use one form of violence, for example, made the use of another more serious form a probable consequence. In turn this led to arguments that use of a knife was not a probable consequence of the use of, say, a common intention to use a baseball bat, or that use of a gun was not a probable consequence of, say, a common intention to use of a knife.
In Edmonds v R  NZSC 159 (20 December 2011) the Supreme Court rejected continued development of this line of case law and directed a return to the words of the statute.
Of course the same issues will continue to arise: was the common intention one which had the probable consequence of the commission of the offence in question? In violent offences the sort of weapon actually used will usually be relevant, but in a way that is directed to the probability of its being used as assessed from the point of view of the defendant who had the original purpose in common with the principal offender.
" The approach of New Zealand courts to common purpose liability must be firmly based on the wording of s 66(2). That section recognises only one relevant level of risk, which is the probability of the offence in issue being committed. If the level of risk recognised by the secondary party is at that standard, it cannot matter that the actual level of risk was greater than was recognised. It follows that there can be no stand alone legal requirement that common purpose liability depends on the party’s knowledge that one or more members of his or her group were armed or, if so, with what weapons. As well, given the wording of s 66(2), there is no scope for a liability test which rests on concepts of fundamental difference associated with the level of danger recognised by the party. All that is necessary is that the level of appreciated risk meets the s 66(2) standard."
From this it is clear, or at least so it seems to me, that (i) the risk recognised by the secondary party is the risk he actually perceived, not the risk he ought to have perceived, (ii) if the secondary party perceives the risk as a "probable consequence" that is sufficient for his liability, (iii) the secondary party may recognise that risk without knowing that the principal party has a weapon, (iv) there are no gradations of the culpable risk - either the preceived risk is of a "probable consequence" or it isn't.
It follows that evidence of the alleged secondary party's knowledge of the possession of a weapon of a different kind from that actually used is relevant not as itself a criterion for liability but rather as material to whether those criteria are met.
This approach to extended secondary liability will apply by analogy to all offences, not just those involving violence. The central issue is whether the alleged secondary party had what amounted to a belief that commission of the actual offence was a probable consequence of the common intention to commit the originally intended offence. It will not be necessary to prove that the alleged secondary party knew that the principal had the means to commit the actual offence, but if he did know that the means existed that would be relevant to assessing whether he had the necessary perception of probable consequence.
As the Court points out (49), it is for the prosecutor to say what the alleged common intention was. The closer the commonly intended offence was to the commission of the offence that was actually committed, the easier it should be to prove that the latter was a probable consequence of commission of the former.
This decision puts extended secondary liability back on the statutory track, away from which the case law had allowed it to drift. However the role of the phrase "in the prosecution of the common purpose" in s 66(2) could still give rise to debate. In committing the offence for which extended secondary liability is contended, did the principal offender go outside - and bring to an end - the prosecution of the common purpose? Had commission of the commonly intended offence been abandoned? This sort of issue is not likely to arise in cases of violence, where the use of force can be seen as a continuum with the commonly intended offence merging with the one for which extended liability is in question. While Edmonds deals with an aspect of extended secondary liability, other problems in applying s 66(2) will need to be addressed.