Expert evidence

[2-1100] Introduction

Last reviewed: June 2023

As to the admissibility of expert evidence, see generally: Pt 3.3 Evidence Act 1995 and note the effect of s 60 of the Act; see also HG v The Queen (1999) 197 CLR 414; Dasreef Pty Ltd v Hawchar (2011) 243 CLR 588 at [30]–[32]; Wood v R (2012) 84 NSWLR 581; Honeysett v The Queen (2014) 253 CLR 122 at [23]–[25]; Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705 at [85]; Taub v R [2017] 95 NSWLR 388 at [19]ff; Criminal Practice and Procedure NSW annotations to [3–s 76]ff; Uniform Evidence Law [1.3.4060]ff; and New Law of Evidence at [76.2]ff.

As to DNA evidence: see Aytugrul v The Queen (2012) 247 CLR 170 at [23]–[24], [30] where it was 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. The “prosecutor’s fallacy” is discussed in R v GK (2001) 53 NSWLR 317; R v Keir [2002] NSWCCA 30 and cf Keir v R [2007] NSWCCA 149. The method by which fingerprint evidence is admitted is discussed in JP v DPP (NSW) [2015] NSWSC 1669 at [39]ff.

As to the role of the jury in relation to expert evidence: see Velevski v The Queen [2002] HCA 4 where there is a discussion as to when it is open to a jury to make a determination between conflicting expert evidence. However, there was no majority decision in respect of whether there was a category of expert evidence that a jury could not resolve: see Velevski v The Queen at [38], [85], [182]. The case does indicate that careful directions need to be given to the jury about expert evidence especially where it is in conflict.

[2-1130] Suggested direction — expert witnesses

Last reviewed: June 2023

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 his or her 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 his or her 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 [his/her] 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 [he/she] had been told by [the accused and/or members of [his/her] family] and that formed part of the history on which [the person] relied to form [his/her] 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 [his/her] 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

Last reviewed: June 2023

In Al-Salmani v R [2023] NSWCCA 83 at [64]–[67] it was 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 choice which necessarily implies an acceptance the expert is capable of answering the questions within the expert’s field of expertise.


In Dirani v R [2021] NSWCCA 202, it was held that while a police expert witness could give evidence of surveillance techniques generally, his mere descriptions of the accused’s behaviour depicted in video recordings and his speculation as to what it meant was not based on any identified expertise and hence was inadmissible: [77]–[92]. At [91]–[92], the court distinguished Kingswell v R (unrep, 2/9/98, NSWCCA), in which an expert police witness gave permissible evidence of an accused’s behaviour by describing the features that gave rise to the opinion.