There are 4 important principles of the criminal law. These principles are set out briefly here and discussed more fully later in this topic and in Court - Criminal Matters.

Innocent until proven guilty (the presumption of innocence)

The basis of our criminal justice system is that a person charged with a criminal offence is considered innocent until proven guilty of the offence. The magistrate, judge or jury (as the case may be) must be satisfied beyond a reasonable doubt that the person is guilty. Where there is a reasonable doubt, the person must be acquitted (found not guilty of the offence).

The fact that a person has been charged does not mean that they are guilty, and any discussion of the charge should make it clear that at this stage the offence is only alleged.

Burden of proof

The prosecution must prove the guilt of the person who is charged with an offence (the defendant). For the defendant to be found guilty of an offence, it must be proved beyond reasonable doubt. It is not up to the defendant to establish their innocence.

This rule applies in all criminal trials, although sometimes is up to the defendant to give evidence of a certain point in the defence case. For example, for conduct that is prohibited "without reasonable excuse", the prosecution must prove that the conduct occurred, but the defendant must show some evidence that they had a reasonable excuse. It is then up to the prosecution to prove that their excuse is not reasonable.

In some cases the burden of proof of a particular defence (such as mental incompetence) may be on the defendant, but then the defence need only be proved on the balance of probabilities, and not beyond a reasonable doubt, as the prosecution must do.

Right to remain silent

Generally a person is not required to answer police questions. However, there are some exceptions to this rule. The main exception is that a police officer can request the name and address of a person found committing an offence, or who the police officer has reasonable cause to suspect has committed, or is about to commit, an offence or of a person who may be able to assist in the investigation of an offence or suspected offence [Summary Offences Act 1953 (SA) s 74A]. In these circumstances a person who refuses to give their name and address, or who gives a false name and address, commits an offence. Drivers of motor vehicles are also required to give their name and address, and that of the owner of the car, as well as some questions in relations to firearms. See Being questioned.

Double jeopardy

The double jeopardy rule protects a person from being punished more than once for the same offending or being placed more than once in jeopardy (at risk) of being convicted for the same conduct. This means that a person who has been charged, tried and acquitted cannot be charged again for the same matter. However, a new trial can be ordered where (for example) an appeal court overturns a conviction or where the first trial resulted in a hung (undecided) jury or a mistrial.

Changes to the Criminal Procedure Act 1921 (SA) mean that double jeopardy no longer applies for serious offences such as murder, manslaughter and aggravated rape, provided certain circumstances are met [see ss 141 - 148]. There are 2 situations in which a person can be re-tried for an offence for which they have previously been acquitted: