As a State offence, unlawful assembly is not defined in an Act of Parliament. It is a “The part of English law traditionally based on common custom and being unwritten. Law which is not equity (q.v.), statute (q.v.), ecclesiastical (church), or civil (i.e. Roman). offence”, whereby it is an offence for there to be an assembly of three or more people with an intent to commit a crime by open force, or with intent to carry out any common purpose, whether lawful or not, in such a manner as to give reasonable people in the vicinity reasonable grounds to fear a breach of the peace.
Also, under section 10 of the Unlawful Assemblies and Processions Act (1958) Vic any person who attends and takes part in an unlawful assembly, as defined by the Act, which includes any political demonstration, is guilty of an offence. This charge is very rarely used, but it does illustrate the power that governments have to stop “political” actions.