Citizen's Arrest.
Check the powers you have for your state.
http://en.wikipedia.org/wiki/Citizen's_arrest#New_South_Wales
For folk in New South Wales.
New South Wales Consolidated Acts
LAW ENFORCEMENT (POWERS AND RESPONSIBILITIES) ACT 2002 - SECT 100
Power of other persons to arrest without warrant
100
Power of other persons to arrest without warrant
(cf Crimes Act 1900 , s 352)
(1) A person (other than a police officer) may, without a warrant, arrest a person if:
(a) the person is in the act of committing an offence under any Act or statutory instrument, or
(b) the person has just committed any such offence, or
(c) the person has committed a serious indictable offence for which the person has not been tried.
(2) A person who arrests another person under this section must, as soon as is reasonably practicable, take the person, and any property found on the person, before an authorised officer to be dealt with according to law.
Victoria
In the Australian state of Victoria, the power to arrest is granted in section 458 of the Crimes Act 1958 (Vic).[4] It states that a person may, without a warrant, arrest a person that they find committing an offence for one or more of the following reasons:
to ensure the appearance of the offender in court, and/or
to preserve public order, and/or
to prevent the continuation or repetition of the offence, or the commission of a further offence, and/or
for the safety or welfare of the public or the offender
A person may also arrest another person if they are instructed to do so by a member of the police force, or if they believe on reasonable grounds that the offender is escaping legal custody.
Section 461 states that if an arrest is made under 458 of the Crimes Act, and is later proven to be false, then the arrest itself won't be considered unlawful if it was done so on reasonable grounds. Section 462A allows any person the right to use force "not disproportionate to the objective as he believes on reasonable grounds to be necessary to prevent the commission, continuance or completion of an indictable offence or to effect or assist in effecting the lawful arrest of a person committing or suspected of committing any offence".