A person is entitled to use such conduct as they genuinely believe is reasonable and necessary for a defensive purpose (that is, in self-defence or in defence of another, or to prevent or end an unlawful imprisonment) [Criminal Law Consolidation Act 1935 (SA) s 15].

If a defendant raises a defence under this section, the defence is taken to have been established unless the prosecution disproves the defence beyond reasonable doubt [s 15(5)].

This will be a complete defence to any offence, including murder, as long as the force used was, in the circumstances as the defendant genuinely believed them to be, reasonably proportionate to the threat the defendant genuinely believed to exist [see for example Zecevic v DPP (1987) 162 CLR 645; [1987] HCA 26]. See however the exception to reasonable proportionality in Home invasion.

It will be a partial defence to murder, reducing the offence to manslaughter, if, despite the defendant believing their actions were reasonable and necessary, the conduct was not reasonably proportionate to the threat that the defendant believed existed [s 15(2)].

Intoxication

From 1 September 2025, a defendant may not rely on a partial defence of self-defence 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 15(2a)].

Drug includes alcohol and any other substance capable of influencing mental functioning [s 15(6)]. 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 15(6)].

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 for a defensive purpose, and whether that particular conduct was reasonable proportionate to a 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)].