When a person is suspected of committing a crime the police have extensive powers to investigate the matter. Most of the police powers are contained 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.
In practice, however, it is usually the police who arrest people and 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 our pamphlet, Police and You, on our Publications page.