Generally, the opinion of a witness is not admissible unless the witness is an expert in the field in which the opinion is given. A doctor may be able to give an opinion as to whether a particular blow may have caused the death of a person, while a non-medical witness cannot give such an opinion as evidence.

There are many court rules (for example, the Joint Criminal Rules 2022 (SA)) and laws surrounding expert opinion evidence and what is allowed by the courts. In particular, it must be directly relevant to the facts in issue and the expert must have expert qualifications or experience in the relevant field.

Lay (non-expert) witnesses may give evidence on matters of fact, or on those things for which the experience of everyday life is sufficient, such as the weather and general identity.

Changes introduced into the Evidence Act 1929 (SA) on 16 December 2024 now permit an Aboriginal person to give evidence about the traditional laws and customs of an Aboriginal group even where such evidence would otherwise be inadmissible as opinion evidence [Evidence Act 1929 (SA) s 34ZA(b)].