Mental Impairment Defence in Victorian Criminal Law

By James Stevens, Director and Solicitor, Go To Court Lawyers. Last reviewed 15 April 2026.

A fundamental principle of the criminal justice system is that a person is held accountable for acts that they committed voluntarily. When a person is mentally impaired, this can amount to a defence to a criminal charge on the basis that the person was not capable of controlling their actions or of knowing that an action was wrong. This article outlines the defence of mental impairment in Victoria.

What is mental impairment?

Mental impairment is a defence under section 20 of the Crimes (Mental Impairment and Fitness to be Tried) Act 1997.

As the Act provides no definition of “mental impairment” the court has often considered the term to mean a “disease of the mind”. The term ‘disease of the mind’ has also been used to describe mental illness or automatism. Mental impairment, however, is not to be mistaken for a medical term; it is a legal term and requires a legal definition.

Section 20 CMIA provides that to successfully rely on mental impairment as a defence, it must be established that at the time of engaging in the conduct constituting an offence:

1) … the accused was suffering from a mental impairment that had the effect that –

(a) he or she did not know the nature and quality of the conduct; or
(b) he or she did not know the conduct was wrong…

… the person must be found not guilty because of mental impairment.

Nature and quality of the conduct

The nature and quality of the conduct is in relation to the accused’s actions and the consequences of that conduct. To satisfy the elements of the defence, the accused must prove that she/he was unable to comprehend that what he/she was doing was unlawful or that there would be consequences for the offending behaviour.

Simply put, the alleged offender must have had no knowledge of any wrongfulness in themselves.

Stated fully, section 20(1)(b) CMIA holds that to successfully make out the defence:

“he or she did not know the conduct was wrong, that is, he or she could not reason with a moderate degree of sense and composure about whether the conduct, as perceived by reasonable people was wrong”.

Whilst the lack of a definition is problematic, the cases of R v Hughes (1989), R v Falconer (1990), and Nolan v R (1997) provided that brain injuries, tumours, and some cases of dissociation and epilepsy are common examples of diseases of the mind. It is therefore from the case law that the definition of mental impairment can begin to be drawn.

Complexities

In Victoria, it is not sufficient for the accused to allege that they lacked a general capacity to control their actions. The accused must prove that a sufficient mental impairment prevailed at the time of the offending behaviour. That is, there must be a correlation between the mental impairment and the offending.

The question of whether the accused has suffered from a mental impairment becomes a question of fact, to be determined by the court on the balance of probabilities.

Despite the substantial number of people affected by mental illness and cognitive impairment in the criminal justice system, this defence is used only infrequently. In addition, this defence is, anecdotally, rarely relied upon during negotiations with the Prosecution.

Where the Defence argues this defence, the Prosecution may move that the matter be referred to the County Court on the basis that either the Magistrates Court does not have the jurisdiction to deal with the complexity of the matter that the Prosecution prefers such matters to be listed in a higher court.

The accused must be advised by counsel that this defence applies only to matters tried summarily in the Magistrates Court. In the Heidelberg Magistrates’ Court, it was stressed that should an accused be charged with an indictable offence which is triable summarily, the matter should remain in the jurisdiction of the Magistrates Court as that Court has the power to dispose of it.

In the event that the matter is transferred for trial in the County Court, one prominent Victorian case, R v Stiles (1990) attempted to address the complexities that lie with raising the defence of mental impairment. At trial, the Prosecution may be directed to prove all the elements of the offence, including that the offending behaviour was a voluntary act, done in the exercise of free will. It is a matter for the Prosecution to prove beyond a reasonable doubt that the accused’s offending behaviour was voluntary.

The Prosecution is required to respond to the defence of mental impairment by putting to the court that the accused was mentally healthy. It then becomes a matter for the Judge to (a) explain mental impairment to the jury; (b) give ‘jury directions’ on how to consider such a defence; and (c) the implications of a not guilty verdict owing to the accused person’s mental impairment.

Taking into consideration the difficulties that this defence gives rise to for all parties, defence lawyers should utilise the opportunity to enter into early negotiations with the Prosecution. This will allow the strength of their case to be tested prior to any request for the matter to be listed in a higher court.

Correlation between mental impairment and offending

For the accused to rely on such a defence, they need to rely upon evidence adduced by an expert witness, such as a psychiatrist, who can confirm their belief that, at the time of the offending behaviour, the accused experienced or suffered from a mental impairment. An affidavit to this effect may also be favourable to the accused during pre-trial discussions with the prosecution.

It is crucial that in making out the mental impairment defence the accused obtains sufficient medical evidence from their attending health professional to draw a necessary correlation between the mental impairment and the offending. For the defence, it is absolutely necessary for the accused to consult a psychiatrist who can provide a detailed report clearly providing the accused’s medical history, a diagnosis of their illness at the time of offending, and a professional medical opinion as to whether the accused suffered mental impairment at the time of the offending.

The accused person’s defence lawyers need to liaise with the medical professional who will be provided with a copy of the accused’s brief for review. This allows the psychiatrist to understand the severity of the allegations made against the accused and to frame their diagnosis, treatment, and report accordingly.

Conclusion

The defence of mental impairment is complex. However, a definition under the CMIA may bring about greater clarification for all involved when trying to determine the guilt of the accused. As the Act is subject to ongoing review, a legislative change is anticipated to clarify the meaning of ‘mental impairment’ in law.

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