In two minds: what happens when an expert witness changes their mind?

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It is without doubt that expert witnesses play a vital role in the dispute resolution process. They provide insights into technical and scientific fields without which the court would often be unable to make a fully informed decision on the case before it. However, expert witnesses present their own set of challenges for courts and legal counsel to grapple with. One such challenge is how to deal with an expert who changes their opinion during the dispute resolution process. This change in opinion could come as a result of, for example, the discovery of new evidence or the realisation of an error in the original report.

How does the court approach the situation where an expert changes their opinion? Is an expert witness entitled to change their opinion? Under what circumstances can this occur? In this blog we delve into the issue of an expert witness changing their mind on expert evidence provided either at trial or in a report. We will also consider a recent case study to demonstrate how a change in expert witness evidence can impact the outcome of a proceeding.

Is an expert witness entitled to change their mind?

In Matthews v SPI Electricity Pty Ltd & Ors (No 10)[1], Derham J noted that while it is “entirely improper” for an expert witness to change their opinion to suit their client’s case as a result of pressure from counsel, experts are permitted, and encouraged, to change their mind when such a change of mind is warranted by the material before the expert.[2]

Similarly, in the recently released Federal Court Practice Notes, the Expert Evidence Practice Note reiterates that “experts should willingly be prepared to change their opinion or make concessions when it is necessary or appropriate to do so, even if doing so would be contrary to any previously held or expressed view of that expert”.[3]

Procedural requirements when an expert changes their mind

Under the Victorian expert witness code of conduct found in Form 44A of the Supreme Court (General Civil Procedure Rules) 2015 (VIC), expert witnesses are provided with specific guidance concerning the required course of action if they change their mind on a material matter:

Where an expert witness has provided to a party (or that party‘s legal representative) a report for the use of the Court, and the expert thereafter changes his or her opinion on a material matter, the expert shall forthwith provide to the party (or that party‘s legal representative) a supplementary report which shall state, specify or provide the information referred to in paragraphs (a), (d), (e), (g), (h), (i) and (j) of clause 3 of this code and, if applicable, paragraph (f) of that clause.

The expert witness code of conduct in Schedule 7 of the Uniform Civil Procedure Rules (2005) (NSW) provides for a similar process in rule 4 as does the Harmonised Expert Witness Code of Conduct in Annexure A of the Federal Court Expert Evidence Practice Note referred to above.

Case Study: change of mind due to errors in the report

In a recent example in the Federal Court, the change of opinion of an expert witness due to an error in their original report had a fundamental impact on the outcome of scheme of arrangement proceedings. The proceedings in question related to a scheme of arrangement for Kasbah Resources Limited, an Australian mineral exploration and development company. The scheme of arrangement had been approved by shareholders and was awaiting court approval.

In the process of contesting the proposed scheme, a small group of shareholders commissioned an expert witness to produce a report opposing the expert report produced by Kasbah. After reviewing the report prepared by the contesting shareholders, Kasbah’s expert witness acknowledged a “fundamental error” in the valuation method used in its report. As a result of this “fundamental error”, the expert witness for Kasbah altered its opinion of the scheme from “fair and reasonable” to “not fair but reasonable”. Following this shift in opinion, the Court did not approve the scheme. On top of this, the Court held that the whole scheme process would need to start again.


It is clear that policy makers encourage expert witnesses to prepare their reports in the most forthcoming manner possible, even if this requires a change in opinion during the trial process. By expressly providing for procedural rules to deal with such a situation, it is clear that experts should keep both their legal team and the court updated of any changes to their initially provided opinions to avoid a lack of compliance with expert witness codes of conduct. However, as expressed by Derham J in Matthews v SPI Electricity Pty Ltd & Ors (No 10)[4], legal counsel should be aware that they are not permitted to coerce an expert witness to alter their already stated opinion to favour their case.

As can be seen by the result in the Kasbah scheme of arrangement proceedings, a change of expert opinion can have devastating effects on the outcome of a proceeding. As such, legal counsel and expert witnesses should remain vigilant in their attempt to reduce the scope for error in the initial report.

[1] [2014] VSC 44

[2] Ibid at [23]

[3] Expert Evidence Practice Note (GNP-EXPT), 4.3.

[4] [2014] VSC 44

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