Evidence of an opinion is generally inadmissible to prove the existence of a fact about the existence of which the opinion was expressed (s 76(1) Evidence Act 1995 (NSW)). The irrelevance of the evidence to the fact in issue and adducing of evidence for some other purpose does not ignite the exclusionary rules. If, however, the evidence is found to be relevant to the fact in issue and another purpose, it will attract protection under an exclusionary rule.
It has been held that:
"Section 76(1) expresses the opinion rule in a way which assumes that evidence of an opinion is tendered to prove the existence of a fact. That manner of casting the rule does not, as might be supposed, elide whatever distinction can be drawn between "opinion" and "fact" or invoke the very difficulty distinction which sometimes is drawn between questions of law and questions of fact.
Rather, the opinion rule is expressed as it is in order to direct attention to why the party tendering the evidence says it is relevant. More particularly, it directs attention to the finding which the tendering party will ask the tribunal of fact. In considering the operation of s 79(1), it is thus necessary to identify why the evidence is relevant: why it is "evidence that, if it were accepted, could rationally affect (directly or indirectly) the assessment of the probability of the existence of a fact in issue in the proceeding". That requires the identification of the fact in issue that the party tendering evidence asserts the opinion proves or assists in proving": Dasreef Pty Ltd v Hawchar (2011) 243 CLR 588, at  per French CJ, Gummow, Hayne, Kiefel and Bell JJ.
It should be noted that the opinion rule in s 76(1) is not confined to evidence of an opinion given by the witness in court, the rule in s 76(1) applies to any "evidence of an out-of-court opinion": Lithgow City Council v Jackson (2011) 244 CLR 352, at .
What is an opinion?
An opinion refers to an inference, belief, or interpretation which is "drawn from observed and communicable data" and which relates to an asserted fact: Allstate Life Insurance Co v ANZ Banking Group Ltd (No 5) (1996) 64 FCR 73, at  and Honeysett v The Queen (2014) 253 CLR 122.
Evidence of an experience or description of evidence is not evidence of an opinion: R v Drolley  NSWCCA 356, at [54-55]. The evidence does not constitute evidence of an opinion until the witness proceeds to draw an inference from their experience. If such an inference is drawn by the witness with the intention of expressing an opinion, the witness must demonstrate why their opinion should not be excluded under the opinion rule. In demonstrating a basis for exclusion, the witness generally seeks to demonstrate that they witnessed the event or matter or that they are objectively qualified to make the opinion asserted.
Exceptions to the opinion rule
Together with an assessment of whether the evidence admitted qualifies as opinion evidence and thereby qualifies under a subsequent exception to the opinion rule, it must also be demonstrated that the evidence is evidence which is relevant within the meaning of s 56 of the Evidence Act. Pursuant to section 55 of the Evidence Act, for evidence to be relevant, the evidence must be capable of rationally affecting (either directly or indirectly) the assessment of the probability of the existence of a fact in issue in the proceeding. Should the opinion be demonstrated to have not been rationally based, it does not satisfy the test of relevance and therefore may not be admitted.
The exceptions to the opinion rule are listed in section 76(2) of the Evidence Act and will be examined below.
Evidence relevant otherwise than as opinion evidence
An exception is provided to evidence "of an opinion that is admitted because it is relevant for a purpose other than proof of the existence of a fact about the existence of which the opinion was expressed": section 77 Evidence Act 1995 (NSW). This exception is provided because, in some circumstances, an opinion may be relevant both as evidence of a fact of an opinion and relevant for another purpose.
The exception to lay opinion prevents the opinion rule from applying to evidence of an opinion expressed by a person if (s 78 Evidence Act):
- The opinion is based on what the person saw, heard, or otherwise perceived about a matter or event, and
- Evidence of the opinion is necessary to obtain an adequate account or understanding of the person's perception of the matter or event.
This provision requires the witness to be present during the matter or event to allow the person to provide an opinion "based on" their perception of the matter or event: Lithgow City Council v Jackson (2011) 244 CLR 352. Section 78(a) is concerned with the substance of the matter of event and whether the opinion was so based, and requires that the opinion falls within the scope of s 78(b), in that, the opinion is necessary to obtain an adequate account or understanding of the person's perception of the matter or event, prior to admission.
Essentially, when evidence of an opinion is sought to be admitted under s 78, the evidence ought to be admitted as evidence of what was perceived (as expressed in s 78(a)) rather than seeking to seeking to adduce inferences from what was perceived about a matter or event. This does not mean that evidence of an opinion cannot be used to draw inferences about perceptions of a matter or event.
It was held in Lithgow City Council v Jackson, at  that:
"It is possible to conclude that a person's opinion is based on what the person perceived without the person providing an exhaustive list of what the person has perceived. It is true, though, that the less the witness or other observer states his or her primary perceptions, the harder it will be for the tendering party to establish the condition of admissibility in s 78(a) and the condition of admissibility in s 78(b)".
If such a recourse is taken, it will be more difficult for the person to demonstrate that the inference drawn from the opinion is based upon the perception of the matter or event and in demonstrating that the inference is necessary to obtain an adequate account or understanding of the matter or event.
Aboriginal and Torres Strait Islander ("ATSI") traditional laws and customs
Section 78A was established to permit members of the ATSI group to give opinion evidence relating to the laws and customs of their people without having to meet the criteria of first having "specialised knowledge" of the ATSI people (s 79). This provision stipulates that:
"The opinion rule does not apply to evidence of an opinion expressed by a member of an Aboriginal and Torres Strait Islander group about the existence or non-existence, or the content, of the traditional laws and customs of the group".
The ATSI person's membership and involvement in the ATSI community is sufficient criteria permitting the ATSI person to provide opinion evidence relating to the laws and customs of the ATSI people.
Section 79 provides an exception to the opinion rule whereby a person provides an opinion relating to a fact in issue that is wholly or substantially based on their specialised knowledge of the fact in issue.
Section 79 stipulates as follows:
(1) If a person has specialised knowledge based on the person's training, study or experience, the opinion rule does not apply to evidence of an opinion of that person that is wholly or substantially based on that knowledge.
(2) To avoid doubt, and without limiting subsection (1):
(a) a reference in that subsection to specialised knowledge includes a reference to specialised knowledge of child development and child behaviour (including specialised knowledge of the impact of sexual abuse on children and their development and behaviour during and following the abuse); and
(b) a reference in that subsection to an opinion of a person includes, if the person has specialised knowledge of the kind referred to in paragraph (a), a reference to an opinion relating to either or both of the following:
(i) the development and behaviour of children generally,
(ii) the development and behaviour of children who have been victims of sexual offences, or offences similar to sexual offences.
Section 79(1) - "Specialised knowledge based on the person's training, study, or experience"
A person is permitted to provide an expert opinion relating to a fact in issue if they can establish that the opinion is based "wholly or substantially on specialised knowledge". For an opinion to be based wholly or substantially on specialised knowledge, it must be established that:
- The person's specialised knowledge is based on the person's training, study, or experience,
- The opinion which is sought and/or provided by the person is "wholly or substantially" based on the specialised knowledge of the person,
- The evidence, for which the expert opinion is required, is relevant to the fact in issue: Jacobs (a pseudonym) v TheQueen  VSCA 285, at . In doing so, the expert is required to disclose the facts upon which the opinion is based: Bugg v Day (1949) 79 CLR 442, at .
- The expert must demonstrate how their conclusion was reached: Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705, at 85.
To ensure that the opinion is admitted as that of an expert, it must be asserted that the knowledge is specialised rather than knowledge which can be generally held in a community. Specialised knowledge has been defined as "knowledge which is outside of persons who have not by training, study, or experience acquired an understanding of the subject matter. It may be of matters that are not of a scientific or technical kind and a person without any formal qualifications may acquire specialised knowledge by experience": Honeysett v The Queen (2014) 253 CLR 133, at .
It will be necessary for the person asserting to bear the specialised knowledge to demonstrate that they have the asserted specialised knowledge: NMFM Property Pty Ltd v Citibank Ltd (No 7) (1999) 161 ALR 576, at  per Lindgren 7. Further, it is imperative that the expert focuses only on the field for which they bear specialised knowledge. The expert may, however, have regard to "matters that are within the knowledge of ordinary persons in formulating his or her opinion": Velevski v The Queen (2002) HCA 4, at  and section 80(b) Evidence Act. The threshold requirement under s 79(1) is that the expert opinion by "wholly or substantially" based on the expert opinion, meaning that the expert may draw on materials external to their own analysis.
It has been widely acknowledged that it is "not uncommon for professional persons or experts in various fields to utilise materials produced by third party specialists for the purpose of assisting them apply their specialised knowledge. That does not mean that the opinion of the former is not based wholly or substantially upon his or her specialised knowledge (the making or production of such materials being proved independently or admitted in legal proceedings)": R v Jung  NSWSC 658, at  per Hall J.
The content of this article is intended to provide a general guide to the subject matter. Specialist advice should be sought about your specific circumstances.