When a person is suspected of committing a crime, the police have extensive powers to investigate the matter. In South Australia, most of the police powers are set out in the Summary Offences Act 1953 (SA).

Technically, everyone has the power to investigate and prevent breaches of the law, and even arrest people who are committing a serious offence. A practical example of a citizen's arrest is a where a store security officer stops a shoplifter caught in the act. See Police security officers, security guards and others for more information.

In practice, however, it is usually the police who arrest people. The law has extended their power so that they can also arrest people who are about to commit an offence, who are planning an offence, or who the police reasonably suspect have committed an offence.

See also The Police and You factsheet and our Duty Solicitor Handbook chapter on Police powers and forensic procedures.