A person is entitled to use such conduct as he or she genuinely believes 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) under s 15 Criminal Law Consolidation Act 1935 (SA).

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.

This will be a complete defence to an 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 (for example, see Zecevic v DPP (1987) 162 CLR 645). 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 his or her actions were necessary, the conduct was not reasonably proportionate to the threat that the defendant believed existed.

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 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)].