The Crown may not refuse to call an expert witnesses it has sought advice from, selecting only those opinions that support their case for tactical reasons, any more than they may refuse to call a witness just because their testimony fails to accord with the prosecution case.

Ashley JA [at 17, agreeing with Neave and Harper JJA]:

17 Even if there had been a significant difference in the opinions of [the two witnesses], in my opinion it does not follow that the Crown was able to pick and choose which witness it called, leaving it to the accused to take up the challenge of calling the other witness. That does not seem to me to accord with the Crown’s obligation of fairly adducing relevant evidence, or with the onus of proof resting on the Crown. No doubt a criminal trial is an adversary proceeding, but that does not mean that it should become a tennis match in which the Crown is able, in effect, to put on the other side of the net a witness whose opinion it has obtained, whose expertise and impartiality are not in issue, but whom it is disinclined to call.

The police informant in Coulson had originally sought an opinion from an independent expert in sleepwalking (sonambulism) to assess the credibility of an explanation put forward by the accused in interview.

The Crown subsequently sought an opinion from another expert and it was he who was named as a witness on the presentment. In pre-trial discussions the defence raised the desirability of the first expert conducting the assessments of the accused he had originally suggested. This was done, but at trial the Crown refused to call the first expert. The Court of Appeal allowed the appeal, ruling that the accused had been deprived of a chance fairly open to him of being acquitted.

Neave and Harper JJA:

61 In our opinion, the calling of [the expert witness) by the Crown was necessary for the presentation in this proceeding of the whole picture. He was appropriately qualified, having practised for 17 years before the appellant’s trial as a specialist in disorders that adversely affect sleep. His evidence was admissible. He was also available. Moreover, his first point of contact with this prosecution came not through the appellant but through the informant who, because of the suggestion that the appellant might be a sleepwalker, thought that (the expert’s] expertise might assist the authorities in their investigation into the complainant’s allegations. [The expert] agreed to review the police brief of evidence, with a view to assessing whether the events as there depicted could (as he put it from the witness box) ‘possibly have been related to sleepwalking’.

Under ss 42 and 416 of the Criminal Procedure Act 2009 the prosecution are obviously obliged to disclose this sort of material regardless of whether they intend to rely on it or not. But the obligation goes beyond mere disclosure to an obligation that, (as it was put in Whitehorn v Queen (1983) 152 CLR 657) ‘all available witnesses should be called whose evidence is necessary to unfold the narrative and give a complete account of the events upon which the prosecution is based’.