Sometimes where there is more than one defendant alleged to be a principal offender (that is, one who personally does the actus reus with the required mens rea) it is difficult to say who did what. This is particularly so where there are a lot of eye witnesses, for it is normal to find that everyone has a different account of what happened. There may be no doubt that an actus reus occurred, but who did what?
This was considered in Huynh v The Queen  HCA 6 (13 March 2013). "Joint enterprise" liability of this kind (which is not to be confused with the term joint enterprise in the context of an exception to the rule against hearsay, nor with similar terminology that is sometimes used in relation to extended secondary liability) depends on proof that the defendant was a party to an agreement to commit the offence that was committed and in addition - for this is not mere conspiracy - that the defendant participated in the offending. Presence when the offending occurred can be sufficient to prove participation. The circumstances may also, as they were in this case, be such that the defendant's presence supports an inference that it was pursuant to the required agreement.
This common law solution to the determination of liability as a principal offender where the facts are vague has a parallel in the interpretation of the statutory criterion, for liability as a principal, of being a person "who actually commits the offence": see for example s 66(1)(a) of the Crimes Act 1961 [NZ]. There is an appeal case, the name and details of which seem still to be subject to a suppression of publication order, so I give only the citation for people with access to databases on which publication is permitted: " NZCA 256". In that case there were several steps needed to complete the actus reus and several defendants were allegedly involved, but no one person did all the steps. In relation to any given defendant, the prosecutor had to prove that that defendant did any of the required steps, that one or more other of the defendants did the other necessary steps, and that the defendant under consideration had mens rea. This was called a distributive application of s 66(1)(a).
Is the distributive approach the same as the joint enterprise liability illustrated by Huynh? Probably yes, although particular facts may produce different results. That would depend on what inference could be drawn from a defendant's presence when another defendant completed a step in the actus reus. The common element is participation in the offending in circumstances where the acts of one defendant are attributed to another.
When will this attribution occur? Policy considerations strongly favour public safety, but there should be clear limits on attribution. For a controversial approach see R v Gnango  UKSC 59,  1 AC 827, discussed here on 18 December 2011, where the majority favoured liability as a principal for murder where the defendant was in a gunfight with the person who, missing him, shot a bystander.
Guidance was given to judges in both Huynh and " NZCA 256" on how to handle complex cases, where there are multiple defendants and different forms of alleged liability. Huynh stresses the importance of getting to the real issue in the case, so that the judge can give a focused direction on the law. The New Zealand case at  invites judges, when faced with disagreement from counsel on the structure and content of a proposed question trail for the jury, to make a minute recording the nature of the dispute and the judge's reasons for adopting the trail ultimately used.