Expert Witnesses: Role in Litigation

Expert witness

What is an expert witness?

An expert witness is a person called to provide an expression of opinion on matters within their particular field of expertise. They do this with specialised knowledge that is based on their training, study or experience, and differ from “lay witnesses” in the sense that ordinarily witnesses may only speak as to what they saw or heard, without expressing their opinions on any those matters. Expert witnesses, by contrast, have expert opinions in their field and the Court relies on those opinions to assist in resolving disputes.


In the Australian Capital Territory, conduct of an expert witness is governed by the Court Procedure Rules 2006 (ACT).

In Schedule 1, titled, “Expert witness code of conduct”, the legislation sets out the permissible conduct of expert witnesses, including:

  1. the application of the code;
  • the general duty expert witnesses have to the Court; and
  • the form in which reports by expert witnesses should be presented, and the directions of the Court that they must comply with.

Of note is the general duty that expert witnesses are required to abide by. Specifically, an expert witness is to refrain from acting as an advocate for either party, and must maintain their duty to the Court as being paramount, as well as to approach matters of relevance to their area of expertise in an impartial manner.

According to section 76 of the Evidence Act 2011 (ACT), expert evidence is admissible if it is based on the expert’s specialized knowledge, gained through their training, study, or experience, and the expert’s opinion is wholly or substantially based on that knowledge. Experts often submit their CV or other relevant qualifications as part of their final report.

Case law

Makita (Australia) Pty Ltd v Sprowles [2001] NSWCA 305, is a leading authority on expert evidence. Here, the NSW Court of Appeal critiqued the plaintiff’s expert for failing to explain how he reached the conclusions he did, especially the facts and assumptions underlying them.  

Nonetheless, it is Heydon JA’s judgment in respect of the parameters under which expert evidence is admissible in Court which has been relied upon in subsequent cases. His Honour stated that for expert evidence to be admissible:

  1. it must be agreed or demonstrated that there is a field of specialised knowledge; 
  • the witness must show specified training, study or experience;
  • the opinion submitted must be wholly and substantially based on the witness’s expert knowledge;
  • to the extent the opinion is based on facts observed by the expert, they must be identified and admissibly proven by the expert;
  • to the extent 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 expert’s evidence must explain how the field of specialised knowledge in which the witness is expert, and on which the opinion is wholly or substantially based, applies to the facts assumed or observed so as to produce the opinion propounded.

These principles formed what became later referred to as the “basis rule”, which is 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.’ Using Makita as authority for their reasoning, the Court slightly modified the admissibility of opinion:

  1. the opinion must pertain to a subject matter that requires expertise;
  • the expert witness must possess specialized knowledge derived from their training, study, or experience, which is relevant to the opinion needed;
  • opinion must be predominantly or entirely based on the expert’s specialized knowledge.

Following these principles is essential to the admissibility of evidence from an expert witness. Today, the ACT Courts continue to apply these principleswhen assessing the admissibility and reliability of expert evidence, in conjunction with the provisions of the Evidence Act 2011 (ACT), and the Court Procedure Rules 2006 (ACT).

If you have any questions relating to the admissibility of expert evidence or the conduct expert witnesses ought to follow as pertaining to relevant case law on evidence and accompanying legislation, please contact BAL Lawyers Litigation and Dispute Resolution team on 02 6274 0999.

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