here - Gordon & Jackson

(1) Evidence of an opinion is not admissible to prove the existence of a fact about the
existence of which the opinion was expressed.
(2) Subsection (1) does not apply to evidence of an opinion contained in a certificate
or other document given or made under regulations made under an Act other than
this Act to the extent to which the regulations provide that the certificate or other
document has evidentiary effect.
An opinion is an inference drawn from observed and communicable data (Allstate Life
Insurance Co v ANZ Banking Group Pty Ltd and Ors (No 32) (1996) 136 ALR 627 at 629 per
Lindgren J). The reason for the exclusion of opinions is pretty straight forward when you
think about it. My opinion as to what happened and your opinion as to what happened may
or may not bear any resemblance to what actually happened and people are normally in
dispute because they have a different opinion as to either what has happened or what
should have happened. Basically, there is only one person in a court room whose opinion
matters, and that is the judge's: it is for the Court to draw inferences, not for parties or
witnesses.
However, Courts have long recognised that sometimes opinions from people experienced in
a particular science or discipline are essential to help them reach a conclusion, and that
without such evidence the Court will be unable to draw appropriate inferences. If you asked
a judge to interpret the raw numbers from a toxicology report, for example, or the
engineering calculations as to the load bearing capacity of a steel beam, they are unlikely to
be able to do so. They would also generally not know whether the inferred result is
significant or not - whether it is right or wrong, normal or exceptional. They need a
toxicologist or an engineer to explain all this to them. There is therefore a special exception
for experts which is set out in section 79. Section 79 reads as follows.
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.
The exception for experts is thus confined to opinions that he or she is able to express
because of their particular expertise. The minute an opinion strays outside that area of
expertise it is inadmissible under section 76 because it is not caught by the exception in
section 79.
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