Under s 15A of the Criminal Law Consolidation Act 1935 (SA) a person is entitled to use such conduct as he or she genuinely believes is necessary and reasonable to defend property from being taken, destroyed, damaged or interfered with; to prevent criminal trespass; to remove a trespasser; or to arrest someone unlawfully at large - as long as the conduct was reasonably proportionate to the threat the defendant genuinely believed to exist, and if the conduct resulted in death, the defendant did not act recklessly or intend to cause death.

However, see also: Home invasion where the requirement of reasonable proportionality does not apply.

It is a partial defence to a charge of murder (reducing the offence to manslaughter) in some circumstances [s 15A(2)].

Criminal trespass in relation to this defence is where someone comes on to a property with the intention of committing an offence (against the person or property) [s 15A(3)].

If a defendant raises this defence then it is for the prosecution to disprove it beyond reasonable doubt [s 15A(4)].

From 29 March 2021

If a defendant asserts that the offence occurred in circumstances of family violence, the questions of whether the defendant genuinely believed that particular conduct was necessary and reasonable to defend property, and whether that particular conduct was reasonable proportionate to the particular threat, are to be determined having regard to any evidence of family violence admitted during the course of trial. For the legislative definitions of circumstances of family violence, and evidence of family violence see sections 34V and 34W of the Evidence Act 1929 (SA) [Criminal Law Consolidation Act 1935 (SA) s 15B(2)-(3)].