Section 99 of the Law Enforcement (Powers and Responsibilities) Act 2002 states that police officers in New South Wales can arrest you without a warrant if:
They suspect on reasonable grounds you are committing or have committed an offence, and they are satisfied the arrest is reasonably necessary:
(a) to stop you continuing the offence or committing another offence,
(b) to stop you from fleeing,
(c) to enable inquiries to establish or confirm your identity,
(d) to ensure you appear before a court,
(e) to obtain property from you that is connected with an offence,
(f) to preserve or prevent the fabrication of evidence,
(g) to prevent harassment or interference of any potential witness,
(h) to protect the safety or welfare of any person, or
(i) due to the nature and seriousness of the offence.