There are many misconceptions around the scope of self-defence, and how it can affect different scenarios. Here are some brief answers to some of the more common questions:
Defending others. In certain scenarios, self-defence may apply if you are acting in defence of a third person, provided you reasonably perceive a threat against him or her, and you act with a defensive purpose. You will still be required to meet the legal test for self-defence, and the court will evaluate the specific scenario to determine if self-defence applies.
Using weapons. The presence or use of weapons, by either you or your attacker, is simply one of the many factors that go into a court’s determination of what is an acceptable defensive response to the attack you faced.
Against police. In the case of police action against you, self-defence is only available if you reasonably believe that the police are acting unlawfully against you, for example by using excessive force. Again, you will still be required to meet the legal test for self-defence, and the court will evaluate the specific scenario to determine if self-defence applies.
Lethal force. The court’s assessment of what is 'reasonable in the circumstances' will depend on many factors. In extreme circumstances, the use of lethal force against another person can be justified if it meets the legal test.
The law in this area is very complex. It is next to impossible to try to navigate this area of the law without having an experienced criminal lawyer examine your unique fact scenario. I have encountered several situations where clients have been charged by police, wrongly, where self-defence clearly applied.