A person is entitled to use such conduct as they genuinely believe 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 [Criminal Law Consolidation Act 1935 (SA) s 15A]. The conduct must be reasonably proportionate to the threat the defendant genuinely believed to exist. If the conduct resulted in death, the defendant must not have acted recklessly or intended to cause death.

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, if the defendant genuinely believed their conduct was necessary and reasonable for a defensive purpose listed in s 15A(1), and they did not intend to cause death, but their conduct was not reasonably proportionate to the threat that the defendant believed existed [s 15A(2)].

If a defendant raises this defence, the prosecution must disprove it beyond reasonable doubt [s 15A(4)].

Intoxication

From 1 September 2025, a defendant may not rely on a partial defence under s 15A(2) to a charge of murder if the defendant's genuine belief that their actions were necessary and reasonable for a defensive purpose was substantially affected by the voluntary and non-therapeutic consumption of a drug [s 15A(2a)].

Drug includes alcohol and any other substance capable of influencing mental functioning [s 15A(5)]. The consumption of a drug will be considered non-therapeutic unless it has been prescribed by and taken in accordance with the directions of a medical practitioner or is available without prescription from a pharmacist and taken as the manufacturer intended [s 15A(5)].

Family violence

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)].