Defence of Mental Impairment

The defence of mental impairment, commonly referred to as insanity, is applicable in all states and territories of Australia. It serves as a legal recourse for a person charged with a criminal offence who claims that they lacked the capacity to understand or control their actions at the time of the alleged crime. This page deals with the defence of mental impairment in various Australian jurisdictions.

Insanity in Queensland

Under section 27 of the Criminal Code 1899 in Queensland, a person has a full defence against any criminal charge if they were unable to understand their actions, regulating their behaviour, or recognise that their behaviour was wrong during the purported criminal act due to a mental illness or infirmity. Mental diseases or infirmities affect an individual’s cognitive functions, including reasoning and memory recall. This encompasses conditions such as brain damage, mental disorders, intellectual disabilities, and short-term mental disorders like concussion.

Mental illness in New South Wales

Section 28 of the Mental Health and Cognitive Impairment Forensic Provisions Act 2020 in New South Wales stipulates that a person is not responsible for an offence if they had a mental health or cognitive impairment at the time of committing the act that resulted in either of the following scenarios:

  • They were unable to comprehend the nature and implications of their actions.
  • They were unable to perceive that their actions were wrong.

Mental impairment in Victoria

Section 20 of the Crimes (Mental Impairment and Unfitness To Be Tried) Act 1997 governs the defence in Victoria. It states that a person has access to a defence if they had a mental impairment that resulted in either of the following circumstances:

  • They were unaware of the nature and significance of their behaviour.
  • They were unaware that their behaviour was wrong.

Insanity in Western Australia

In WA, the defence of insanity is established under section 27 of the Criminal Law Act Consolidation Act, which affords protection to an accused person who had a mental impairment that rendered them incapable of comprehending their actions, regulating their behaviour, or recognising that their actions were wrongful. Additionally, the legislation in Western Australia provides a defence for persons who were experiencing delusions. Such accused persons will be held accountable to the same extent that they would have been if their delusional beliefs had been correct.

Mental impairment in the ACT

Section 28 of the Criminal Code 2002 sets out the defence in the ACT. The ACT definition of mental impairment encompasses conditions such as intellectual disability, temporary and permanent mental illnesses, senility, brain damage, and severe personality disorders.

Expert evidence

When the defence of mental impairment is invoked, the court usually hears evidence from psychiatrists and/or psychologists as to whether the accused was mentally impaired at the time of the alleged offence. The court will evaluate all the evidence and factors before making a decision on whether the accused is not guilty due to mental impairment. Although expert witnesses may provide their opinions, the court is not obligated to accept these opinions.

Effect of a verdict of not guilty because of mental impairment

If a person is acquitted on the ground of mental impairment, they cannot be sentenced by the criminal courts and must be dealt with through alternative means. Some states have courts and tribunals that are dedicated to dealing with persons who have been acquitted due to mental impairment. These people may be ordered to undergo psychiatric care or treatment, either in a community or in a detention setting. The terms of the order will be periodically reviewed to ensure that they remain appropriate.

Mental impairment vs fitness to plead

The question of whether a person has a defence of mental impairment is separate from the issue of their fitness to plead, although both may arise in the same case. The defence of mental impairment concerns the individual’s ability to comprehend and control their actions at the time of the alleged crime. Their mental state at the time of the trial is not relevant. In contrast, the issue of fitness to plead relates to the person’s capability to comprehend and participate in court proceedings, regardless of their mental state at the time of the alleged offence.

If you require legal advice or representation in any legal matter, please contact Go to Court Lawyers.


Fernanda Dahlstrom

Fernanda Dahlstrom has a Bachelor of Laws from Latrobe University, a Graduate Diploma in Legal Practice from the College of Law, a Bachelor of Arts from the University of Melbourne and a Master of Arts (Writing and Literature) from Deakin University. Fernanda practised law for eight years, working in criminal defence, child protection and domestic violence law in the Northern Territory. She also practised in family law after moving to Brisbane in 2016.
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