The "reasonable excuse" or "lawful excuse" defence is enshrined in our common law and, in some cases, in our statute law in Australia.
For example, federal laws make it an offence to trespass, or refuse to leave Commonwealth land when directed. However, all this is subject to whether you had a reasonable excuse. The same applies to offensive conduct and language offences under NSW law. Lawful excuse is a defence to a charge of trespass under the (NSW) Inclosed Lands Protection Act 1901.
Our common law has been built up over many years and inherited from the English common law. Our common law is influenced by court decisions in England, and (to a lesser degree), by decisions of common law of countries including Canada, New Zealand and the United States.
Three court cases that relied upon the "reasonable excuse" or "lawful excuse" defence are outlined here.
Each case was about people taking action in order to prevent a greater crime...