It is an offence for anyone, alone or together with others, to:
willfully and without lawful authority “beset” any premises, whether public or private, for the purpose and with the effect of obstructing, hindering or impeding by an assembly of people any person’s lawful right to enter, use or leave such premises (section 52(1A) Summary Offences Act1966 (Vic)).
In plain language, this means: it is illegal to block all the entry and exits to a building so that no-one can get in or out of the building.
To be found guilty of this offence, the police must prove that you obstructed the entry and exits so that it was not possible for a person to enter or leave the premises. (See: HRLC case note on Victoria Police v Anderson  VMC 22 (Max Brenner Case)).
In other words, obstructing one or more doorways while others were accessible would not be enough to find protesters guilty of besetting.
The maximum penalty is a 15 penalty units fine or three months’ jail.
You should seek legal advice about the penalty you may face if you are found guilty of besetting.