“Pleading Insanity” in South Australia – Defences of Mental Impairment


Hollywood has led to a popular belief that one may “plead insanity” when accused of a serious offence and easily be duly acquitted. The position in South Australia is far more nuanced and  mental impairment defences are often very complex and quite difficult. A defence of mental impairment is governed by Part 8A of the Criminal Law Consolidation Act. The standard to which an accused person will need to meet to be considered mentally incompetent to have committed an offence is set out at Section 269C of the Act.

269C – Mental Competence

A person is mentally incompetent to commit an offence if, at the time of the conduct alleged to give rise to the offence, a person is suffering from a mental impairment and, in consequence of the mental impairment-

  • does not know the nature and quality of the conduct; or
  • does not know that the conduct is wrong; or
  • is unable to control the conduct.

This standard has evolved out of the venerable M’Naughten Rule – a reference to M’Naughten’s Case of 1843; the first truly complex “insanity” defence in history in which Daniel M’Naughten mistakenly shot public servant Edward Drummond believing him to be the British Prime Minister. The House of Lords in M’Naughten’s case developed a central test that essentially asked the question;-

Did the defendant know what they were doing and if so, did they know if was wrong?

The development of this rule into the current position that forms the basis of a mental impairment defence in South Australia, Section 269C, is evident. Happily, however, advances in psychiatric medicine and social attitudes have moved with the jurisprudence on the subject to recognising a wide variety of mental illnesses that may render an accused unable to be found guilty having regard to the requirements of the Act.

Difficulties can arise where the mental impairment giving rise to a defence is a temporary one; for example, where an accused has suffered a momentary psychosis, it can be a difficult forensic exercise to prove the impairment suffered by the defendant, as they may have very little recollection to report to a forensic psychiatrist. Further difficulties will arise given that the mental impairment may have completely passed by the time of arrest. This can be contrasted with the position where an accused has a clear and ongoing mental impairment that is plainly evident following his arrest, and indeed during the forensic evidence gathering procedures undertaken by both the defence and the prosecution in the months following an arrest. In such a case, the mental impairment of a defendant can be much more simply assessed by forensic psychiatrist.

A second misconception that often arises in relation to mental impairment defences is that an acquittal on the basis of Part 8A will mean that a defendant will “walk free”. Whilst it is the case that being found not guilty pursuant to a successful mental impairment defence will mean that the accused is, in effect acquitted, the Court, on the basis of having the objective elements of the defence in question being proved, will then set what is known as a limiting term. A limiting term will be a “sentence” but will be set by way of reference to either the accepted sentencing range for a particular offence or sentences for analogous fact situations. These factors will be assessed in tandem with submissions made by respective Counsel. The accused will then be subject to conditions that can be extremely wide in scope. In some instances, the accused will be virtually unrestricted whereas other defendants may conversely  be subject to a requirement of mental health detention for a lengthy period of time, in an approved facility (but not a gaol). The license conditions are reviewed regularly by law and the progression of persons subject to a limiting term will be monitored by psychiatrist. A person detained pursuant to a license or subject to other restrictions may make a application in the Supreme Court for less restrictive conditions on the basis of expert reports indicating their suitability.

If you are a loved one of a person accused of an offence, and you believe a defence may be available on the basis of mental impairment, contact Culshaw Miller Criminal Lawyers on 0418 421 153 or by e-mail at alex.scott@culshawmiller.com.au.