Physical intervention in the act of self-defence is a very grey legal area. In many respects, the legal proceedings following a self-defence event in which someone is seriously injured or killed are very subjective processes. The court has to come to a decision on whether the level of force used in the act of self-defence was justifiable or not based on available information. For some background reading, I refer you to the Wikipedia article on Self-Defence in Australia: https://en.wikipedia.org/wiki/Self-defence_(Australia)