I have included a case summary and discussion below.
Hawchar was an employee who performed stone cutting for Dasreef Pty Ltd (Dasreef). Hawchar developed silicolysis during his employment with Dasreef and issued proceedings in the Dust Diseases Tribunal. Hawchar relied on an expert report provided by Dr Basden, and in his report Dr Basden gave evidence that while he worked, Hawchar could have been exposed to silica dust concentrations at least 1000 times greater than the permissible limit. Dr Basden based this opinion on the pleadings, a photograph of the mask Hawchar used, a photograph of Hawchar demonstrating the use of a cutting disc and a photograph of the Dasreef premises.
Dasreef objected to this evidence at trial and the Tribunal (implicitly) admitted the evidence. The Tribunal used this evidence to make calculations about the amount of dust Hawchar was exposed to. The objection to the evidence was that the plaintiff was attempting to use Dr Basden's report to demonstrate the actual exposure of Hawchar to silica dust, whereas Dr Basden in evidence said that he was not giving an opinion about the volume of dust that the plaintiff was exposed to at the time, but giving an opinion about the appropriate precautions to be used when exposed to a particular concentration of silica dust.
An appeal was made by Dasreef to the Court of Appeal against the admission of the report, which was dismissed. Dasreef appealed to the High Court. The appeal was upheld and the report was ruled inadmissible.
The High Court discussed s79 in detail. The majority (French CJ, Gummow, Hayen, Crennan, Kiefel and Bell JJ) summarised s79(1) at  to  as follows:
- Section 79(1) of the Evidence Act must be understood in its statutory context. Section 76(1) of the Evidence Act provides that "[e]vidence of an opinion is not admissible to prove the existence of a fact about the existence of which the opinion was expressed". That exclusionary rule is referred to in the Evidence Act as "the opinion rule". Subsequent provisions of the Evidence Act provide a number of exceptions to the opinion rule. Section 79(1) provides that:
"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."
- 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 difficult distinction which sometimes is drawn between questions of law and questions of fact. It does not confine an expert witness to expressing opinions about matters 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 to make. 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 identification of the fact in issue that the party tendering the evidence asserts the opinion proves or assists in proving.
- To be admissible under s 79(1) the evidence that is tendered must satisfy two criteria. The first is that the witness who gives the evidence "has specialised knowledge based on the person's training, study or experience"; the second is that the opinion expressed in evidence by the witness "is wholly or substantially based on that knowledge". The complaint which Dasreef made at trial, on appeal to the Court of Appeal and on appeal to this Court was that Dr Basden did not express an opinion about the numerical or quantitative level of exposure to respirable silica encountered by Mr Hawchar in working for Dasreef that was an opinion based on any specialised knowledge Dr Basden had that was based on his training, study or experience.
- In order for Dr Basden to proffer an admissible opinion about the numerical or quantitative level of Mr Hawchar's exposure to silica dust it would have been necessary for the party tendering his evidence to demonstrate first that Dr Basden had specialised knowledge based on his training, study or experience that permitted him to measure or estimate the amount of respirable silica to which a worker undertaking the relevant work would be exposed in the conditions in which the worker was undertaking the work. Secondly, it would have been necessary for the party tendering the evidence to demonstrate that the opinion which Dr Basden expressed about Mr Hawchar's exposure was wholly or substantially based on that knowledge.
- In this case, demonstration of those matters could come only from evidence given by Dr Basden. That is why, in HG v The Queen, Gleeson CJ pointed out that, "[b]y directing attention to whether an opinion is wholly or substantially based on specialised knowledge based on training, study or experience, [s 79] requires that the opinion is presented in a form which makes it possible to answer that question".
- It should be unnecessary, but it is nonetheless important, to emphasise that what was said by Gleeson CJ in HG (and later by Heydon JA in the Court of Appeal in Makita (Australia) Pty Ltd v Sprowles) is to be read with one basic proposition at the forefront of consideration. The admissibility of opinion evidence is to be determined by application of the requirements of the Evidence Act rather than by any attempt to parse and analyse particular statements in decided cases divorced from the context in which those statements were made. Accepting that to be so, it remains useful to record that it is ordinarily the case, as Heydon JA said in Makita, that "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". The way in which s 79(1) is drafted necessarily makes the description of these requirements very long. But that is not to say that the requirements cannot be met in many, perhaps most, cases very quickly and easily. That a specialist medical practitioner expressing a diagnostic opinion in his or her relevant field of specialisation is applying "specialised knowledge" based on his or her "training, study or experience", being an opinion "wholly or substantially based" on that "specialised knowledge", will require little explicit articulation or amplification once the witness has described his or her qualifications and experience, and has identified the subject matter about which the opinion is proffered.
- Dr Basden gave evidence of his training, study and experience. He did not give evidence asserting that his training, his study or his experience permitted him to provide anything more than what he called a "ballpark" figure estimating the amount of respirable silica dust to which a worker using an angle grinder would be exposed if that worker was using it in the manner depicted in the photograph of Mr Hawchar or a video recording Dr Basden was shown. Indeed, in his written report, Dr Basden had pointed out that he had seen the use of an angle grinder in this way only once before. And he gave no evidence that he had then, or on any other occasion, measured directly, or sought to calculate inferentially, the amount of respirable dust to which such an operator was or would be exposed.
- There was, in these circumstances, no footing on which the primary judge could conclude that a numerical or quantitative opinion expressed by Dr Basden was wholly or substantially based on specialised knowledge based on training, study or experience.
- Contrary to submissions on behalf of Mr Hawchar, this analysis does not seek to introduce what has been called "the basis rule": a rule by which opinion evidence is to be excluded unless the factual bases upon which the opinion is proffered are established by other evidence. Whether that rule formed part of the common law of evidence need not be examined. It may be accepted that the Law Reform Commission's interim report on evidence denied the existence of such a common law rule and expressed the intention to refrain from including a basis rule in the legislation the Commission proposed and which was later enacted as the Evidence Act 1995 (Cth) and the Evidence Act 1995 (NSW). What has been called the basis rule is a rule directed to the facts of the particular case about which an expert is asked to proffer an opinion and the facts upon which the expert relies to form the opinion expressed. The point which is now made is a point about connecting the opinion expressed by a witness with the witness's specialised knowledge based on training, study or experience.
- A failure to demonstrate that an opinion expressed by a witness is based on the witness's specialised knowledge based on training, study or experience is a matter that goes to the admissibility of the evidence, not its weight. To observe, as the Court of Appeal did, that what Dr Basden said about the volume of respirable dust to which Mr Hawchar was exposed over time was "an estimate" that was "contestable and inexact" no doubt did direct attention to its worth and its weight. But more importantly, it directed attention to what exactly Dr Basden was saying in his evidence and to whether any numerical or quantitative assessment he proffered was admissible. And if, as the Court of Appeal observed, his opinion on that matter lacked reasoning, the absence of reasoning pointed (in this case, inexorably) to the lack of any sufficient connection between a numerical or quantitative assessment or estimate and relevant specialised knowledge.
- Dr Basden's evidence was not admissible to found the calculation made by the primary judge of the level of respirable dust to which Mr Hawchar was exposed.