Opinion evidence 

Witnesses are ordinarily called to tell a court what they perceived, not what they concluded from it. The distinction between fact and opinion sounds simple, but applying it to real testimony — where perception and inference blend together in ordinary speech — is one of the more subtle skills in evidence law.

The general rule

The opinion rule provides that evidence of an opinion — an inference or conclusion drawn from observed or assumed facts, rather than a direct account of those facts — is generally not admissible to prove the existence of the fact about which the opinion is held. The underlying concern is that drawing inferences from the primary facts is the fact-finder's job. A witness who is allowed simply to assert their conclusion, rather than describing what they actually saw, heard, or otherwise perceived, risks substituting their own judgment for the fact-finder's, and risks doing so without the fact-finder being able to test the reasoning that produced the conclusion.

In practice this means witnesses are expected to describe events, conditions, and observations in factual terms, leaving the drawing of inferences from that primary evidence to the tribunal of fact. The line is not always sharp — ordinary language routinely compresses observation and inference together — which is part of why the exceptions matter so much in practice.

The expert opinion exception

The most significant exception is for expert opinion evidence. A person with specialised knowledge, based on their training, study or experience, may give an opinion that is wholly or substantially based on that specialised knowledge, where the opinion falls within the field of that expertise and assists the fact-finder to understand matters outside ordinary lay experience — the requirement Clark v Ryan (1960) 103 CLR 486¹ insists on genuinely, not merely nominally. The rationale is the mirror image of the general rule's concern: where a subject genuinely requires specialised understanding — technical, scientific, medical, or otherwise — the fact-finder is assisted, not usurped, by hearing a suitably qualified opinion, provided the expert's reasoning and the basis for the opinion are exposed so that it can be tested and evaluated rather than simply accepted on authority.

In the uniform Evidence Act jurisdictions these requirements are statutory, and Dasreef Pty Ltd v Hawchar [2011] HCA 21² states them as the two criteria s 79(1) imposes. At [32] the joint reasons of French CJ, Gummow, Hayne, Crennan, Kiefel and Bell JJ put it this way: the witness who gives the evidence must have "specialised knowledge based on the person's training, study or experience", and the opinion expressed must be "wholly or substantially based on that knowledge".

Establishing this exception typically requires showing that the witness has the requisite specialised knowledge, that the opinion is genuinely based on that knowledge rather than on speculation or advocacy, and that the opinion stays within the boundaries of the expert's actual field. An expert opinion that strays into matters outside the witness's demonstrated expertise, or that simply asserts a conclusion on the ultimate issue without exposing the reasoning that supports it, is vulnerable to challenge on this ground.

Lay opinion evidence

A more limited exception permits lay opinion evidence — opinion given by an ordinary witness without specialised knowledge — in confined circumstances, typically where the opinion is about a matter within ordinary human experience and is necessary to convey what the witness perceived. Common examples include a witness describing that a person appeared intoxicated, that a vehicle was travelling fast, or that a person seemed upset: these are, in form, conclusions, but they are conclusions drawn from a compound of observations that would be cumbersome or artificial to break down into pure sensory description. The exception is narrow and does not extend to opinions on complex or technical matters, or to a lay witness's view on the ultimate question the court must decide.

Applying this in a problem question

1. Identify whether the disputed testimony is an account of what was perceived, or a conclusion drawn from it — the opinion rule is engaged only by the latter.
2. If it is opinion, ask whether the witness has specialised knowledge based on training, study or experience, and whether the opinion is genuinely based on and confined to that knowledge.
3. If the witness is not an expert, consider whether the lay opinion exception applies — an ordinary matter within common experience, expressed as a natural way of conveying a perception.
4. Be alert to opinion evidence that strays onto the ultimate issue the fact-finder must decide, which invites closer scrutiny even where a witness is qualified generally.
5. Remember the opinion rule is only one hurdle — the evidence must also be relevant and clear any other applicable exclusionary rule.