Expert evidence
[2-1100] Introduction
Under s 79(1) of the Evidence Act 1995, evidence of an opinion based wholly or substantially on a person’s specialised knowledge, which is based on their training, study or experience, is admissible as an exception to the opinion rule in s 76. See generally: Criminal Practice and Procedure NSW annotations to [3–s 76]ff; and Uniform Evidence Law (20th edn) [EA.76.30]ff.
In Honeysett v The Queen (2014) 253 CLR 122 at [23], the High Court said:
Specialised knowledge is knowledge which is outside that 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.
Specialised knowledge includes knowledge of child development and child behaviour (including the impact of sexual abuse on children and their development and behaviour during and following the abuse): s 79(2); see also the exception to the credibility rule in s 108C discussed in BQ v The King [2024] HCA 29.
The opinion, and the facts on which it is based, must be clearly delineated: HG v The Queen (1999) 197 CLR 414 at [39]. The evidence must show the process of reasoning behind the opinion, and explain how the field of specialised knowledge applies to the particular facts or assumptions to produce the opinion: Dasreef Pty Ltd v Hawchar (2011) 243 CLR 588 at [37] (quoting Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705 at [85]), and [128]–[130].
In some cases, “little explicit articulation or amplification” will be required to demonstrate a connection between the specialised knowledge and the expert opinion: Dasreef Pty Ltd v Hawchar at [37]; see JP v DPP (NSW) [2015] NSWSC 1669 at [39]ff in relation to the admission of fingerprint evidence.
The requirement that the opinion be “wholly or substantially based” on specialised knowledge reflects that it will sometimes be difficult to separate the specialised knowledge from "observations and knowledge of everyday affairs and events": Honeysett v The Queen at [24], quoting Velevski v The Queen [2002] HCA 4 at [158].
The truth of the facts or assumptions underlying the expert opinion is a matter to be evaluated by the fact-finder, and does not need to be proved in order for the opinion to be admissible under s 79: Taub v R [2017] 95 NSWLR 388 at [19]ff. An opinion which lacks cogency of reasoning may be excluded under ss 135 or 137, but once admitted, the reliability of the opinion is a matter of weight: see Lang v The Queen (2023) 278 CLR 323 per Kiefel CJ and Gageler J at [15]–[17] and Jagot J at [436].
Once admitted, the opinion can also become evidence of the existence of a fact about which the opinion was expressed by the operation of s 60, unless limited under s 136: R v Welsh (1996) 90 A Crim R 364 at 367–369.
Failure of an expert witness to comply with the code of conduct in Sch 7 of the Uniform Civil Procedure Rules 2005 will not necessarily render the evidence inadmissible, although it may be excluded under ss 135 or 137: Wood v R (2012) 84 NSWLR 581 at [719]–[729]; Chen v The Queen [2018] NSWCCA 106.
Where expert opinions are provided in terms of statistical probability, careful directions or warnings may be required to avoid the danger of impermissible reasoning or misuse of the evidence: see R v GK (2001) 53 NSWLR 317; R v Keir [2002] NSWCCA 30 and cf Keir v R [2007] NSWCCA 149.
In Aytugrul v The Queen (2012) 247 CLR 170 at [23]–[24] and [30] the majority held that it was not erroneous to direct a jury on the basis of an exclusion percentage where a frequency ratio had also been given and where the relationship between the two figures had been explained.
Careful directions need to be given to the jury about expert evidence, especially where it is in conflict. In Dasreef Pty Ltd v Hawchar, the majority stated at [92], “choosing between conflicting experts depends in part on “impressiveness and cogency of reasoning” (quoting Monroe Australia Pty Ltd v Campbell (1995) 65 SASR 16 at 27, quoting Sotiroulis v Kosac (1978) 80 LSJS 112 at 115). In Velevski v The Queen, there was no majority decision in respect of whether there was a category of competing expert evidence that a jury could not resolve: see [38], [85], [182].
[2-1130] Suggested direction — expert witnesses
In this case, [CD and EF] have been called as expert witnesses. An expert witness is a person who has specialised knowledge based on their training, study or experience. Unlike other witnesses, a witness with such specialised knowledge may express an opinion on matters within their particular area of expertise. Other witnesses may speak only as to facts, that is, what they saw or heard, and are not permitted to express their opinions.
The value of any expert opinion very much depends on the reliability and accuracy of the material which the expert used to reach their opinion. It also depends on the degree to which the expert analysed the material upon which the opinion was based and the skill and experience brought to bear in formulating the opinion given. Experts can differ in the level and degree of their experience, training and study, yet each can still be an expert qualified to give an opinion where that opinion is based on that witness's specialised knowledge.
Expert evidence is admitted to provide you with … [specify, for example, scientific/medical/accountancy/etc] information and an opinion on a particular topic which is within the witness's expertise, but which is likely to be outside the experience and knowledge of the average lay person.
The expert evidence is before you as part of all the evidence to assist you in determining … [set out the particular aspect(s), for example, the mental condition of the accused; whether the accused’s act was voluntary; the nature and effect of a series of financial transactions; the properties of a particular drug and its effects; the mechanical condition of a truck, etc, as the case may be]. You should bear in mind that if, having given the matter careful consideration, you do not accept the evidence of the [expert(s)], you do not have to act upon it. This is particularly so where the facts upon which the opinion is based do not accord with the facts as you find them to be. You are also, to a degree, entitled to take into account your common sense and your own experiences if they are relevant to the issue upon which the expert evidence relates.
[Where there is a conflict between the experts, add
In this case, there is a conflict between the expert evidence of [AB] called on behalf of the Crown and [CD] who was called on behalf of the accused. It goes to the issue of … [specify the issue(s)]. It is not a case of simply choosing between their evidence as a matter of simple preference. [Where the accused has the onus of proof, emphasise the relevant standard of proof and how it operates in relation to the expert evidence].
It is for you to decide whose evidence and whose opinion you accept in whole or in part, or whose evidence you reject altogether. You should remember that this evidence relates only to part of the case, and that while it may be of assistance to you in reaching a verdict, you must reach your verdict having considered all the evidence.
[There has been no challenge to the qualifications of any of the expert witnesses, all of whom you may think are well qualified].]
[Summarise the arguments of the parties as to why a particular expert should be preferred or discuss with the jury, matters relevant to the resolution of the evidence, such as the reliability of the information relied upon and the level of expertise of a particular witness.]
In resolving the conflict in the expert evidence, you are entitled to consider that particular evidence in the context of all of the evidence that is before you, and especially that part of the evidence which may have a bearing on the acceptance or otherwise of a particular opinion.]
[Where there has been no challenge to the expert evidence either in cross-examination or by calling evidence to the contrary, add
The expert evidence has not been challenged. Accordingly, if it is not inherently unbelievable, you would need to have a good reason to reject it — for example, because it does not fit with other facts which you have found proved.]
[Where there is conflict as to the facts or assumptions underlying the opinion, add
The expert evidence of [AB], called on behalf of the Crown, relating to … [specify points], appears to be based on facts which [AB] has been told, or on assumptions which [AB] has been asked to make [specify the facts or assumptions]. You should analyse the evidence of [AB] and determine the extent to which their opinion depends upon the facts or assumptions being correct.
If the opinion is based upon facts which you are satisfied have been proved, or assumptions that you are satisfied are valid, then it is a matter for you to consider whether the opinion based upon those facts or assumptions is correct. On the other hand, if you decide the facts have not been proved, or the assumptions are not valid, then any opinion based upon them is of no assistance because it has no foundation. If that is the case, the opinion should be disregarded.
[This direction can be modified where the opinion is relied upon by the defence, bearing in mind which party bears the onus of proof in respect of the issue, the subject of the evidence].]
[Where the expert witness relies on statements by the accused and/or others, and they do not give evidence, and no direction is given under s 136 limiting the use to be made of that material, add
The expert [CD] recounted what they had been told by [the accused and/or members of the accused’s family] and that formed part of the history on which [CD] relied to form their opinion. That is why that material was admitted despite the fact that it was hearsay evidence, that is, evidence of statements made outside the courtroom by persons not called as witnesses before you. However, that material is evidence before you and you are entitled to rely on it, not merely as statements made to the expert and upon which to evaluate their opinion, but also as evidence of the truth of the facts contained in those statements. However, I warn you that as those statements are hearsay they may be unreliable. The person or persons making those statements did not give evidence before you and, therefore, could not be tested by cross-examination [give other reasons for the possible unreliability of the statements depending upon the facts and circumstances of the particular case].]
[2-1140] Notes
- 1.
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In Al-Salmani v R [2023] NSWCCA 83 at [64]–[67] the Court held that it will be a rare case where responsive answers by an expert to a cross-examiner’s questions would be objectionable, and it is incumbent on counsel to raise any objections to an expert straying from their expertise at the trial. This is because cross examining counsel can confine questioning to the field of the witness’s expertise and choosing to go beyond that field is a forensic decision which necessarily implies an acceptance that the expert is capable of answering the questions within the expert’s field of expertise: [66].
- 2.
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The mere reading of various papers in a field of discipline by a person with no relevant prior training, study or experience will not constitute “study” within the meaning of s 79(1): AJ v R (2022) 110 NSWLR 339 at [77].
- 3.
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In JS v DPP (NSW) [2026] NSWCA 58, the Court held that an expert certificate could be admitted pursuant to the requirements in s 177 without requiring separate compliance with s 79. If notice is given and the expert is required to give evidence under ss 177(5), 79 will apply to any opinion they express: [45]–[46].
- 4.
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An expert witness can be a person who has acquired specialised knowledge “ad hoc” or, in other words, for the very purpose of the legal proceedings in which the opinion evidence is relied upon: Nasrallah v R [2015] NSWCCA 188 at [21]; Ali v R [2022] NSWCCA 199 at [50]. In Ali v R, the Court held that evidence of an interpreter that a “male voice” speaking in Hindi in several intercepted telephone conversations was the same person was admissible under s 79 (citing Butera v DPP (Vic) (1987) 164 CLR 180). This was because the interpreter’s opinion was based on their specialised knowledge of the Hindi language, and the nuances of the pitch, tone and accent of the voice which were specific to that language: [52]–[57].
- 5.
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Evidence of identification by a purported ad hoc expert will be irrelevant and therefore inadmissible if it is based on material that is not different, in any substantial way, from what is available to the jury: Smith v The Queen (2001) 206 CLR 650 at [10]–[12]. In these situations, the witness will be in no better position than the jury to make an assessment: Smith v The Queen at [9]; Ali v R at [57].