A witness may only tell the court what they saw or heard or otherwise witnessed, and not what someone else told them about what happened. This is hearsay evidence. Hearsay evidence cannot be used to establish (prove) the content of an out-of-court statement.

For example, a witness can tell the court "I saw Jill push Jack down the hill" and this can be used as evidence to establish that Jill did in fact push Jack down the hill. A witness may not be called to say "I wasn't there at the time, but Tom Piper told me that he saw Jill push Jack down the hill". Tom Piper should instead give evidence of his observation and recollection, and this can then be tested by the defendant's lawyer.

Whether particular evidence is hearsay or not is often a difficult question to answer. This is a complex area of law and there are many exceptions to the general rule, such as admissions or statements against self-interest, and certain statements made by persons who have since died and are unable to give evidence.

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 hearsay evidence [Evidence Act 1929 (SA) s 34ZA(a)].