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The Defence of Mental Impairment

Every state and territory of Australia has a version of the defence of mental impairment. This defence, also known as insanity, applies when a person charged with a criminal offence argues that they are not guilty because they were not capable of understanding or controlling their actions at the time of the alleged offence. This page deals with the defence of mental impairment in different Australian jurisdictions.

Insanity in Queensland

Under section 27 of the Criminal Code 1899 in Queensland, a person has a full defence to any criminal charge if they were unable to understand their behaviour, control their behaviour, or understand that they ought to not do the behaviour during the alleged criminal act because of a mental disease or infirmity.

A mental disease or infirmity is a condition that impacts the functions of a person’s mind, including the ability to reason or recall. This includes brain damage, mental illnesses, intellectual disabilities, or temporary mental disorders like a concussion.

Mental illness in New South Wales

In New South Wales, under section 28 of the Mental Health and Cognitive Impairment Forensic Provisions Act 2020, a person is not guilty of an offence if at the time of carrying out an act they had a mental health impairment or cognitive impairment that meant that they:

  • Did not know the nature and quality of the act; or
  • Did not know that the act was wrong.

Mental impairment in Victoria

In Victoria, the defence is contained in section 20 of the Crimes (Mental Impairment and Unfitness To Be Tried) Act 1997. Under that provision, a person has a full defence if they were suffering from a mental impairment that had the effect that they:

  • Did not know the nature and quality of the conduct;
  • Did not know that the conduct was wrong.

Insanity in Western Australia

In WA, the defence of insanity is contained in section 27 of the Criminal Law Act Consolidation Act. It provides a defence for a person who was mentally impaired so as not to be able to understand what they were doing, control their actions or understand that they should not do the actions.

The Western Australian legislation also provides a defence to an accused person who is suffering from delusions. Such a person will be held criminally responsible only to the extent that they would have been responsible if what they believed to exist had been the real state of affairs.

Mental impairment in the ACT

In the ACT, the defence of mental impairment is set out in section 28 of the Criminal Code 2002. Mental impairment in the Act is defined as including intellectual disability, short-term and long-term mental illnesses, senility, brain damage and severe personality disorders.

Expert evidence

In cases where the defence of mental impairment is raised, the court generally hears testimony from psychiatrists and/or psychologists. Expert witnesses may offer testimony regarding whether the accused was mentally impaired at the time of the alleged offence.

The court will determine whether the accused is not guilty based on mental impairment after hearing all the evidence and considering all the circumstances surrounding the alleged offence. It will not necessarily accept the opinions of the expert witnesses.

Effect of a verdict of not guilty because of mental impairment

A person who is found not guilty on the basis of mental impairment cannot be sentenced and must be dealt with outside of the criminal justice system. Different states have different courts and tribunals for dealing with those found not guilty by reason of mental impairment.

A person who is not guilty because of mental impairment may be ordered to receive psychiatric treatment or care either in the community or in detention. Such an order will be reviewed periodically to ensure that the conditions of the order are still appropriate.

Mental impairment vs fitness to plead

The issue of whether a person had a defence of mental impairment is distinct from the question of their fitness to plead, although both may arise in the same case.

The mental impairment defence pertains to the accused’s capacity to comprehend and regulate their conduct at the time of the purported offense. Their mental state at the time of the trial is irrelevant.

On the other hand, the question of fitness to plead deals with the accused’s ability to understand and engage in the 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 Armstrong Legal.

Fernanda Dahlstrom

This article was written by Fernanda Dahlstrom

Fernanda Dahlstrom has a Bachelor of Laws, a Bachelor of Arts and a Graduate Diploma in Legal Practice. She has also completed a Master’s in Writing and Literature. Fernanda practised law for eight years, working in criminal defence, child protection and domestic violence law in the Northern Territory and in family law in Queensland.

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