ON 14 September 2001, the NSW Court of Appeal delivered Makita (Australia) Pty Ltd v Sprowles [2001] NSWCA 305 (14 September 2001).

The common law rules regarding the admissibility of opinion evidence were summarised by Heydon JA as follows:

• An expert has a duty to provide the trial court with criteria to allow the evaluation of the validity of the expert’s conclusions (at [59]).
• The trial court is to decide whether or not to accept the conclusions.
• The intellectual basis or essential integers of the expert opinion must be explained to the trial court to allow it to arrived at an independent assessment of the opinions and their values (at [68], [71] and [79]).
• The trial court must give weight to the opinions in the same way as for the evidence of non-expert witnesses (at [82]).
• The expert’s opinion is to be based on facts, either proved by the expert or disclosed as assumptions of fact that form the basis of the opinion [at 64].
• the opinion will be admissible and material if other admissible evidence establishes that the assumptions are sufficiently likely even though not completely precise.
• The expert witness is not an advocate. The paramount is to be impartial to the court. This duty overrides its obligation to the engaging party. The expert witness is not an advocate (at [77]).
• The expert witness is to assist the trial court in determining a matter in issue, but the court must weigh and determine the probabilities of the fact on the whole of the evidence (at [67]).
• The expert’s particular expertise is to be applied to the assumed or proven facts in order to come to his or her opinion (at[59]).

Per Heydon JA (at [85]):

“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 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 R [1999] HCA 2; (1999) 197 CLR 414 (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.”

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