Evidence that constitutes an inference, evaluation, interpretation or belief rather than observed fact. Opinion evidence is inadmissible in court proceedings as a general rule, although there are some exceptions. Opinion evidence is generally excluded because it has the potential to mislead the court (i.e. the jury) despite being irrelevant and extraneous material. It is generally understood that the only value judgment or opinion allowed in a court should be the one formed by the jury/judge at the close of proceedings, and that should be based on solely on the tribunal of fact before it.
Section 76 of the Evidence Act (NSW) excludes opinion evidence from admissibility:
“Evidence of an opinion is not admissible to prove the existence of a fact about the existence of which the opinion was expressed.”
‘Opinion’ is not defined in the Act, but case law has confirmed the definition of opinion as being ‘an inference from observed and communicable data’ (Seltsam Pty Ltd v McNeill  NSWCA, Lithgow City Council v Jackson (2011) CLR).
The primary exception to the rule against opinion evidence is expert opinion (section 79). In determining the weight to be given to expert opinion evidence, Heydon JA in the case of Makita v Sprowles, stated that an expert witness need furnish the trier of fact with criteria enabling evaluation of the validity of the expert’s conclusions. This case established the test for expert opinion evidence.