In NSW, the law recognises that a person can protect themselves when they are being physically attacked or are faced with a threat of physical violence. However, the extent to which violence can be used in ‘self-defence’ depends on the circumstances and the extent of the threat faced.
Under section 418 of the Crimes Act 1900, a person is not criminally responsible for an offence if the person carries out the offence in self-defence.
A person carries out an offence in self-defence if the person believes their actions are necessary
In addition, the conduct must be a reasonable response to the circumstances as the person perceives them.
There are two questions that must be considered by a court when self-defence is raised:
The first question is determined by considering the personal characteristics of the accused and the situation as the accused perceived it at the time: R v Katarzynski  NSWSC 613. In other terms, this is an entirely subjective assessment. Therefore, matters such as the age of the accused, their gender, or the state of his or her health may be considered by the court.
The second question is determined by an entirely objective assessment of the proportionality of the accused’s response. It is important to note that that accused need not have had reasonable grounds for their belief that it was necessary to act in the way they did in order to defend themselves, it is sufficient if they genuinely held that belief
In any criminal proceedings in self-defence is raised, the prosecution has the onus of proving, beyond reasonable doubt, that the person did not carry out the conduct in self-defence.
If the possibility that the offence was committed in self-defence cannot be excluded, the accused must be found not guilty.
If a person raises self-defence, they do not have to give evidence before the court: Colosimo v DPP, however, other evidence must support self-defence being raised: Mencarious v R.
If a person uses force that involves the intentional or reckless infliction of death to protect property or to prevent criminal trespass, per section 420 the defence of self-defence is not available.
Per section 421, if a person uses force that causes death and that conduct is not a reasonable response in the circumstances, but the person believes the conduct is necessary to defend themselves or another person; or to prevent the unlawful deprivation of their liberty, they can be found not criminally responsible for murder but criminally responsible for manslaughter.
When a person is charged with murder, a conviction for manslaughter is ‘open’ as an alternative. What that means is that if the prosecution proves beyond a reasonable doubt that what the accused did was not a reasonable response in the circumstances then the accused can be found not guilty of murder but guilty of manslaughter. Alternatively, if it is found that the prosecution has failed to prove beyond a reasonable doubt that the accused was acting in self-defence, the accused may be found not guilty of murder and not guilty of manslaughter.
If the accused was voluntarily intoxicated at the time of the offence, their intoxication must be taken into account in assessing whether they believed the conduct was necessary in response to the circumstances as they perceived them: R v Katarzynski  NSWSC 613.
However, intoxication is not to be taken into account in determining whether the accused’s response to the threat was reasonable: R v Katarzynski  NSWSC 613.