- the person called as an expert has specialised knowledge;
- that specialised knowledge is based on the person's training, study or experience; and
- the opinion of the person which is adduced in evidence is wholly or substantially based on that knowledge.
The most referenced consideration of s79 is the judgment of Heydon JA (as His Honour then was) in Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705. In Makita at the trial level, an employee was suing an employer for injuries resulting from slipping on the employer's stairs. The employee relied on an expert which opined that the tread on the stairs was slippery at the time of the accident. That opinion was based on tests performed more than 9 years after the accident on the stairs. Further, evidence was given that the stairs were used by the employee prior to the accident, and by others at all times, without incident. The Trial Judge admitted the report and, on appeal, the NSW Court of Appeal held that the Trial Judge had erred in accepting the evidence.
At  Heydon JA set out a summary of the rules for admissibility of expert evidence under s79, as follows:
In short, if evidence tendered as expert opinion evidence is to be admissible, it 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 specified 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 witness demonstrates that by reason of specified 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 by the expert, and so far as the opinion is based on "assumed" or "accepted" facts, they must be identified and 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, but, to use Gleeson CJ's characterisation of the evidence in HG v The Queen (at 428 ), on "a combination of speculation, inference, personal and second-hand views as to the credibility of the complainant, and a process of reasoning which went well beyond the field of expertise".In Idoport Pty Ltd & Anor v NAB Ltd & Ors  NSWSC 123 Einstein J considered the admissibility of expert opinion evidence under s79 and referred to extra curial commentary of Heydon JA (as His Honour then was) within a paper delivered at a seminar in 2000 dealing with aspects of the Evidence Act. The commentary, according to Einstein J, 'conveniently identifies and elucidates the relevant requirements under the following 7 headings' (at ):
- There must be a field of specialised knowledge and the witness must identify it.
- The witness must have expertise in an aspect of that field, and must identify it.
- The opinion proffered must be substantially based on the expertise of the witness and the witness must identify it.
- Any factual assumptions underlying the witness's opinion must be clearly identified and articulated.
- Any factual observations made by the witness which underly the witness' opinion must be clearly identified and articulated, and the observations must have been sufficiently detailed to form a satisfactory basis for the opinion.
- If the witness relies on a combination of factual assumptions and factual observations, they must be identified.
- The witness must explain how the knowledge on which the witness is an expert applies to the facts assumed or observations made so as to produce the opinion propounded.