This can be broken down into the following three mandatory pre-requisites:
- the person has 'specialised knowledge';
- that 'specialised knowledge' is based on 'the person's training, study or experience'; and
- the opinion is 'wholly or substantially based on that knowledge'.
As set out by Vickery J at  in Roads Corporation v Love  VSC 537 (26 November 2010), the general principle, as stated by the New South Wales Court of Appeal in Makita (Australia) Pty Ltd v Sprowles is that:
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, 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”. [Citations omitted]Examples of expert opinion evidence (randomly taken from Odgers, Uniform Evidence Law in Victoria (2010) at [1.3.4260] include:
- A psychologist may be able to express opinions about human behaviour but not as to why a person told lies in a particular context.
- A medical practitioner may be able to express opinions regarding the causes of particular injuries but not about how much force would have been required to cause the injuries or whether the injuries were inflicted deliberately.
- An expert on plants may be able to identify a plant as cannabbis but not estimate its age.
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