To be admitted or not to be admitted, that is the question which many practitioners are confronted with, upon the initial review of sizeable briefs served by the prosecution.

Best practice demands a thorough analysis to be conducted by the practitioner in relation to the veracity of all statements, the use of the evidence sought to be admitted, and the type of evidence sought to be admitted by the prosecution.

One such categorisation of evidence requiring scrutiny is the opinion evidence of an expert.


Generally, "evidence of an opinion is not admissible to prove the existence of a fact about the existence of which the opinion was expressed": s 76 Evidence Act 1995 (NSW) ("EA").

This rule is subjected to many exceptions, one such exception is if a person has specialised knowledge pursuant to s 79 EA.

Section 79(1) Evidence Act 1995 (NSW) states as follows:

"If a person has specialised knowledge based on a 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".

Prior to evidence of a person's "opinion" being admitted pursuant to s 79(1) EA, it is imperative that it is established that the opinion upon which the evidence is based upon the person's training, study, or experience and that such opinion is based "wholly or substantially on [that person's specialised] knowledge".

It is insufficient for an assumption to be made that simply because the person holds a particular qualification in the field of interest that they are necessarily an expert.

Expert reports must be carefully assessed to ensure that they are relevant to the facts in issue, that the person holds expertise, that the report does not contain any unfair prejudice to the defendant, that there is no potentially inadmissible hearsay evidence and to ensure that the "expert person" has adequately shown their reasoning in providing their expert opinion.

Relevance of the expert opinion

It must be sufficiently established that the expert opinion is relevant to the facts in issue: s 56 EA.

The expert opinion must contain a :

"... precise identification of the issue, to which the proposed opinion evidence is relevant, is necessary in order to determine whether, in fact, the evidence is probative of that fact in issue, and, if so, whether the witness, who is to give the evidence, is qualified to express that opinion, and whether that opinion is wholly or substantially based on the witness' expert knowledge": Jacobs (a Pseudonym) v The Queen [2019] VSCA 285 at [47].

There is an inherent need for the party seeking to adduce the expert opinion evidence to establish a nexus between the facts upon which the opinions are based and the reasoning which led or supported the opinions based: Canon Finance Australia Limited v Reliance Medical Practice Pty Ltd & Ors (No 2) [2018] NSWSC 1374.

This is essential as it seeks to demonstrate the relevance of the expert opinion evidence to the facts which are presently in issue.

Should the expert opinion not contain any reasoning relating to the facts in issue, it is, in short, inadmissible to be adduced as an expert opinion: Jacobs at [47].

Further, it is important to note that should the expert evidence include any material which is prejudicial to the defendant it may be excluded under s 137 EA.

The court must determine whether the 'probative value [of the evidence sought to be adduced by the prosecution] is outweighed by the danger of unfair prejudice to the defendant".

This exclusion is only available in criminal proceedings and so for present purposes is relevant only for criminal lawyers.

The Basis Rule

There exists an additional requirement at common law which must be met by the expert prior to admission of the expert evidence.

The expert must establish that their opinion was properly founded upon the evidence and/or facts which are duly available to the expert: Dasreef Pty Ltd v Hawchar (2011) 243 CLR 588 at [41] and Ramsay v Watson (1961) 108 CLR 642 at [649].

In obiter dicta, Heydon JA set out the requirements which experts must adhere to, to ensure compliance with s 79(1) in Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705 at [85]:

"In short, if evidence tendered as expert opinion evidence is to be admissible:

  • There must be agreed or demonstrated that there is a field of "specialised knowledge",
  • There must be an identified aspect of that field in which the witness demonstrates that by reason of specialised training, study, or experience, the witness has become an expert,
  • The opinion proffered must be "wholly or substantially based on the witness's expert knowledge",
  • So far as the opinion is based on facts "observed" by the expert, they must be identified and admissibly proved in some other way,
  • It must be established that the facts on which the opinion is based form a proper foundation for it,
  • And the opinion of an expert requires demonstration or examination of the scientific or other intellectual basis of the conclusions reached: that is, the expert's evidence must explain how the field of "specialised knowledge" in which the witness is expert by reason of "training, study or experience", and on which the opinion is "wholly or substantially" based", applies to the facts assumed or observed so as to produce the opinion propounded.

If all these matters are not made explicit, it is not possible to be sure whether the opinion is based wholly or substantially on the expert's specialised knowledge. If the court cannot be sure of that, the evidence is strictly speaking not admissible, and, so far as it is admissible, of diminished weight. And an attempt to make the basis of the opinion explicit may reveal that it is not based on specialised expert knowledge.

This reasoning has been accepted in many later cases and is now widely used in determining whether the expert's opinion is "wholly or substantially" based on the expert's specialised knowledge: Dasreef.

Failure of the expert to adequately address the basis of their opinion will no doubt mean that the evidence will face a question of relevance under s 56 EA as it will create difficulties for assessing the ability of the evidence in affecting the probability of the existence of the fact in issue.

Specialised knowledge

The person upon which the opinion is based must sufficiently demonstrate that their opinion is wholly or substantially based upon their "specialised knowledge" of the field.

This is a mandatory requirement of admission and must be based upon the expert's "training, study, or experience in the subject matter and must not extend beyond the bounds of the expert's specialised knowledge.

Specialised knowledge has been widely discussed by judicial officers since the commencement of this provision and the issue of its definition has been resolved in Honeysett v The Queen (2014) 253 CLR 122 at [23]:

"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".

It is knowledge which is not generally held by the community.

Knowledge sought to be adduced which is a matter of "common knowledge" may be sought to be admitted under s 80 EA.

Whilst the expert must only rely upon subject matter which is within their own field of expertise, 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 [28] and section 80(b) Evidence Act.

The threshold requirement under s 79(1) is that the expert opinion is "wholly or substantially" based on the specialised knowledge.

The imposition of such a requirement emplaces a duty onto the expert to provide a transparent account of the reasoning process adopted in their report which has ultimately led to the formation of the opinion in the subsequent report: HG v The Queen (1999) 197 CLR 414 at [39]-[44], Dasreef Pty Ltd v Hawchar (2011) 243 CLR 588 at [42] and Morgan v The Queen [2011] NSWCCA 257 at [140].

It is through this process of reasoning that the expert will be able to demonstrate that the report is substantially based upon their specialised knowledge of the subject matter and will be able to assist the court in resolution of an contentions relating primarily to the fact in issue.

Finally, an expert may assert their opinion which is substantially based on their specialised knowledge notwithstanding their utilisation of external material in making their opinion.

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 [2006] NSWSC 658, at [57] per Hall J.


In many cases, experts will utilise out-of-court representations to base their opinion.

Importantly, such representations are not captured by the hearsay rule under s 59 EA.

This is primarily because the representations contained in the expert report are not being adduced to "prove the existence of a fact that it can reasonably be supposed that the person intended to assert by the representation", instead, the representation is relevant for another purpose - to explain the bases upon which the expert opinion is formulated: s 60 EA.

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.