[39] The opinions of Mr McCombie were never expressed in admissible form. An expert whose opinion is sought to be tendered should differentiate between the assumed facts upon which the opinion is based, and the opinion in question. Argument in this Court proceeded upon the basis that it was possible to identify from Mr McCombie's written report some facts which he either observed or accepted, and which could be distinguished from his expressions of expert opinion. Even so, the provisions of s 79 will often have the practical effect of emphasising the need for attention to requirements of form. By directing attention to whether an opinion is wholly or substantially based on specialised knowledge based on training, study or experience, the section requires that the opinion is presented in a form which makes it possible to answer that question.
[40] Mr McCombie's report referred to a number of matters he took into account in reaching the conclusions he expressed: things he was told by the complainant, by her mother, and by the general practitioner who referred the complainant for assessment; his training as a psychologist; his experience in counselling victims of sexual abuse; and his knowledge of patterns of behaviour of disturbed children. It is not in dispute that psychology is a field of specialised knowledge, and that a psychologist may be in a position to express an opinion based on his or her specialised knowledge as a psychologist. However, the witness had to identify the expertise he could bring to bear and as Clark v Ryan illustrates, his opinions had to be related to his expertise.
[41] If all that Mr McCombie had said was that, based on his study, training and experience, he considered that the behaviour of the complainant during 1992 and 1993, as recounted to him by others, appeared to be inconsistent with her having been sexually abused during that time, (the plausibility of such a proposition is not now in issue), then that might have been one thing. It would have required identification of the facts he was assuming to be true, so that they could be measured against the evidence; and it would have required or invited demonstration or examination of the scientific basis of the conclusion. However, that was not what the defence wanted from him; if it were, no question of s 409B would have arisen. What defence counsel wanted was evidence of his opinion that, although the complainant had been abused, the abuse had occurred back in 1987 when, for a period of a month, she was in the custody of her father, and that it was the father who was the abuser. That opinion was not shown to have been based, either wholly or substantially, on Mr McCombie's specialised knowledge as a psychologist. On the contrary, a reading of his report, and his evidence at the committal, reveals that it was based on a combination of speculation, inference, personal and second-hand views as to the credibility of the complainant, and a process of reasoning which went well beyond the field of expertise of a psychologist. He did not put to the complainant, for her comment, the suggestion that she had been abused by her father; the complainant told him she could not remember her father. He does not appear to have considered or investigated the possibility of abuse by some third party. He appears to have inferred, for no apparent reason, that the words "stop it daddy", attributed to the complainant by her mother, referred to sexual as distinct from some other form of abuse.
[42] Logically, there were a number of competing possibilities. The complainant may have been sexually abused by nobody; she may have been abused as she claimed, by the appellant; she may have been abused by her father; she may have been abused by both her father and the appellant; she may have been abused by some person or persons unknown. It was not demonstrated, and it is unlikely, that it is within the field of expertise of a psychologist to form and express an opinion as to which of those alternatives was to be preferred.
[43] To paraphrase what was said by Dixon CJ in Clark v Ryan about the expert witness in that case, the evidence the defence sought to lead from Mr McCombie really amounted to putting from the witness box the inferences and hypotheses on which the defence case wished to rely.
[44] This was not a trial by jury, but in trials before judges alone, as well as in trials by jury, it is important that the opinions of expert witnesses be confined, in accordance with s 79, to opinions which are wholly or substantially based on their specialised knowledge. Experts who venture "opinions", (sometimes merely their own inference of fact), outside their field of specialised knowledge may invest those opinions with a spurious appearance of authority, and legitimate processes of fact-finding may be subverted. The opinions which Mr McCombie was to be invited to express appear to provide a good example of the mischief which is to be avoided.
[45] The evidence in question was not admissible as opinion evidence. (citations omitted)