Generally, every person is presumed to be mentally competent to commit a criminal offence [Criminal Law Consolidation Act 1935 (SA) s 269D].

A person charged with an offence may raise a defence of mental incompetence if, at the time of the alleged conduct, they were suffering from a mental impairment such that they:

[Criminal Law Consolidation Act 1935 (SA) s 269C(1)]

Mental impairment can include a mental illness, an intellectual disability or a disability or impairment of the mind resulting from senility [s 269A].

The defence of mental impairment recognises that the criminal law responds to and punishes physical conduct that occurs with a criminal intent or guilty mind. A person who commits an offence but who was not aware of their conduct or did not understand that it was wrong ought not be held criminally responsible and liable to criminal sanction. See also Elements of a Criminal Offence.

A defendant who raises mental impairment as a defence to alleged conduct must prove it on the balance of probabilities.

A defendant, their lawyer or the prosecution must inform the court as soon as possible if they have reason to believe that the defendant may have been mentally incompetent to commit the charged offending [Joint Criminal Rules 2022 (SA) r 67.2]. The defendant's lawyer (if they have one) must also inquire into the defendant's mental competence as soon as possible if concerns are raised.

Where the defence of mental impairment is raised, the question of the defendant's mental competence must be separated from the remainder of the trial [s 269E(1)]. The trial judge may choose to start with the objective (physical) elements of the offence or the mental competence of the defendant [s 269E(2)].

A defendant found to have committed the objective elements of an offence but who successfully relies on a defence of mental impairment will have a finding recorded of "conduct proved but not criminally responsible due to mental incompetence". Prior to 14 July 2025, such a defendant would be found "not guilty by reason of mental impairment". This terminology changed on 14 July 2025 to better reflect the defendant's level of responsibility in the conduct.

A person found mentally incompetent to have committed an offence, in circumstances where the trial judge is satisfied, on the balance of probabilities, that the mental impairment was substantially caused by self-induced intoxication, may be dealt with under Part 8 of the Criminal Law Consolidation Act 1935 (SA) rather than Part 8A [s 269C(2), see Intoxication]. Intoxication will be regarded as self-induced if it results from the recreational use of a drug or the combined effect of the therapeutic consumption of a drug and the recreational use of a drug [s 269A(2a), (2b)]. The court may nevertheless order that a defendant whose mental impairment was substantially caused by self-induced intoxication be dealt with as mental impairment under Part 8A, having regard to the circumstances of the intoxication and the interests of justice [s 269C(3)].

A person found to have committed the objective elements of an offence but who was mentally incompetent at the time of the offending may be released unconditionally, released on licence for a specified period or detained in secure psychiatric care [ss 269NB and 269O]. It is a condition of every licence that the defendant be prohibited from possessing firearms and ammunition. The court may also direct a defendant to surrender a firearm [ss 269NC and 269OA]. When determining whether to release a defendant, either unconditionally or on a licence, the paramount consideration must be the safety of the community [ss 269NA and 269NI].

Supervision orders are governed by Chapter 2 Part 2 of the Uniform Special Statutory Rules 2022 (SA).