The Honourable Justice Chesterman, formerly of the Queensland Court of Appeal, once defined the role of the expert witness as: “to educate or inform the court about the relevant aspects of the witness’s specialty to enable the court itself to access evidence, which, without that tuition, the court would be unable to do”. 
In most circumstances, the court does not admit mere opinion into evidence, as indicated by the opinion rule in s76 of Evidence Act (Cth) 1995:
(1) “Evidence of an opinion is not admissible to prove the existence of a fact about the existence of which the opinion was expressed.”
However, s79 of the Evidence Act makes an exception for opinions based solely on specialised knowledge and training:
(1) “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.”
Though a large majority of experts have academic and/or professional qualifications from an appropriate established faculty, formal qualification is not always a necessity to confer an individual the title of expert. Nevertheless, expert opinion on certain fields of expertise undergo greater scrutiny. The Court is unlikely, for example, to admit medical opinion from a witness who is not registered to practice medicine or not accredited by an established educational institution.
The Rule in Makita
Expert opinion cannot be admitted to Court based on the opinion-giver’s expertise alone. The opinion must be drawn from facts that the expert has either observed or assumed, and must be substantially based on the expert’s specialised knowledge. 
For instance, in Makita (Australia) Pty Ltd v Sprowles,  the initial verdict went in favour of the employee, Ms Sprowles, who injured herself after slipping on a set of stairs in the parking lot above the building. The expert stated the stairs were, indeed, slippery but did not provide a factual basis for his opinion. The NSW Court of Appeal eventually overturned the verdict to favour the plaintiff, due to the failure of the plaintiff’s expert to explain the facts and assumptions which formed the basis of his conclusion.
The admissibility of expert evidence is further subject to two overriding requirements:
- The evidence must be relevant to the proceedings (s56 of Evidence Act); and
- Even if relevant, the Court may refuse to admit the evidence if its probative value is outweighed by other considerations such as:
– being unfairly prejudicial to a party;
– being misleading or confusing; or
– will cause or result in an undue waste of time (s135 of Evidence Act).
The Expert Witness Code of Conduct
A common concern held by judges is the issue of experts acting as ‘hired guns’ and not providing evidence that is objective and impartial.
For this reason, expert witnesses are also required to read and abide by the appropriate Expert Witness Code of Conduct under the jurisdiction in which they are retained. The Code of Conduct varies between jurisdictions, but two universal themes are retained in all versions of the Code:
- The expert’s primary duty is to the Court, not to the party retaining the expert; and
- The expert should provide independent assistance to the Court, by giving an objective, impartial opinion in relation to matters wholly within his or her area of expertise.
Given the second requirement, experts are excluded from providing an opinion on areas beyond their specialised knowledge. It is important to ensure that the expert does not venture beyond their area of expertise. In HG v The Queen  Gleeson CJ observed that:
“Experts who venture opinions (sometimes merely their own inference of fact), outside their field of specialised knowledge may invest those opinions with a spurious appearance of authority, and legitimate processes of fact-finding may be subverted.” 
This includes the possibility of the expert going beyond the scope of their retainer, and addressing the ultimate issue, instead of abiding by their duty to assist the court. As the High Court stated in Ramsay v Watson,  the jury “… may be assisted by the … evidence. But they are not simply to transfer their task to the witnesses.’ 
In summary, for someone to be accepted as an expert witness they need to have the appropriate specialised knowledge, based on training, study, or experience and they must make certain that their opinion is based substantially on that specialised knowledge. They also need to be briefed on their responsibilities to the Court and ensure that their opinion is drawn from admissible facts that they have assumed or observed. Finally, they must remain within their field of specialised knowledge and not venture beyond the realm of their expertise.
 Justice Chesterman RFD, “The Accountant as Expert Witness”, Conference Paper – Institute of Chartered Accountants (12 March 2000), p 2.
 Miiko Kumar, ‘Admissibility of Expert Evidence: Proving the Basis for an Expert’s Opinion’.
Sydney Law Review, VOL 33:427 2011 at 427.
 (2001) NSWCA 305.
 (1999) 197 CLR 414.
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 (1961) 108 CLR 642 (at 645).
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