When does this defence arise?
The Defence Has Subjective And Objective Requirements
The High Court in the case of Zecevic v. Director of Public Prosecutions (1987) 162 CLR 645 set out 2 elements to the defence:
1. The Accused must have believed at the time that s/he committed the relevant act that what s/he was doing was necessary (known as the “subjective element”); and
2. That belief must have been based on reasonable grounds (the “objective” element).
The Accused Has the Burden of Raising the Defence
An accused has the obligation to adduce evidence that raises self-defence as a fact in issue, however once self-defence is raised, the prosecution must disprove the accused acted in self-defence reasonable doubt.
The Proportionality of the Accused's Response Not the Only Consideration
In determining the actions of the accused proportionality is always relevant, but not necessarily decisive of whether the accused acted in self-defence (Zecevic v Director of Public Prosecutions (1987) 162 CLR 645; R v Portelli (2004) 148 A Crim R 282 (Vic CA).
If proportionality is a live issue for a contested hearing or trial, the prosecution must establish that the force used by the accused was “out of all proportion” or plainly disproportionate” to any attack which the accused could reasonably have believed was threatened by the complainant.
The law does not expect any accused to precisely consider the response that is proportionate in the circumstances. (R v Said  VSCA 244).
Failure to retreat from an imminent assault is not a requirement but is relevant in determining whether the objective element of the defence. That is, whether the accused believed on reasonable grounds that her/his actions were necessary.
Factors Relevant to the Reasonableness of the Accused's Belief
Case law has established that in determining whether an accused’s belief was based on reasonable grounds a variety of matters are relevant including:
- Proportionality of the accused’s response.
- Prior conduct of the victim
- Personal characteristics of the accused, and whether they were susceptible to disordered beliefs they may have held; and more generally their state of mind (distress, fear) that preceded their actions
- The relationship of the parties involved
- Surrounding circumstances
- Failure to retreat
- Intoxication of an accused