The Defence of Insanity (WA)

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

The defence of insanity provides that an individual cannot be convicted of a criminal offence if, at the time of the act, they were suffering from a severe mental illness that rendered them unable to comprehend their actions or exercise control over them. This defence serves to strike a balance between holding individuals responsible for their behaviour and acknowledging that a person should not be accountable for actions committed while not in control of their faculties. This page deals with how the insanity defence works in Western Australia as well as how it relates to the issue of fitness for trial.

Defence of insanity vs fitness for trial

The defence of insanity is distinct from the issue of an accused person’s fitness to stand trial. However, in certain cases where mental unsoundness is at issue, both of these inquiries may be necessary.

The insanity defence focuses on the accused's mental state during the alleged offence, while the fitness for trial inquiry pertains to the accused's mental state at the time of the court proceedings.

It is possible for an individual to have a viable insanity defence but still be fit to stand trial at the time of the hearing. Conversely, an individual may lack an insanity defence but still be unfit for trial during the court proceedings.

Fitness for trial

Section 9 of the Criminal Law (Mentally Impaired Accused) Act 1996 outlines the criteria for an accused person to be deemed mentally unfit to stand trial, including a lack of understanding of the nature of the charge, entering a plea, the purpose of the trial, their right to challenge jurors, or inability to follow the course of a trial or defend themselves properly. Until proven otherwise, an accused person is presumed to be mentally fit to stand trial.

If a person’s fitness for trial is in doubt, the court typically orders a ‘fitness to plead’ report, prepared by a qualified professional after assessing the accused person, before deciding if the person is fit for trial.

If the accused is found to be fit for trial, the matter proceeds. If they are unfit, the matter cannot proceed and the person must be dealt with outside the criminal justice system.

How is the insanity defence defined?

Under section 27(1) of the Criminal Code 1913, an individual cannot be held criminally responsible for an action or failure to act if, at the time of the act or omission, they were so mentally impaired that they lacked the ability to understand what they were doing, control their behaviour, or recognize that their actions were wrong.

The act also provides a defence for individuals suffering from delusions. If an individual's mental state at the time of an act or omission is influenced by delusions regarding specific matters, and they do not qualify for an insanity defence, they will only be held accountable for their actions to the same extent as they would be if the beliefs induced by the delusions had been correct.

Presumption of sanity

Under section 26 of the Criminal Code 1913, there is a presumption that all accused persons are mentally sound. Therefore, the burden of proof lies with the defence if they accused wishes to invoke the insanity defence and establish that the accused was not mentally sound at the time of the alleged offence.

The defence must be established on the balance of probabilities.

The determination of whether an accused person can avail themselves of the defence under section 27 of the Act is a factual question that can only be resolved after a thorough examination of all the evidence presented during a hearing.

Expert evidence

To acquit a person based on the insanity defence, the court must be convinced that the accused was mentally unsound at the time of the alleged offence. This is usually done using on expert evidence.

Effect of the defence of insanity

Once the insanity defence is successfully proven, the accused will be acquitted of the offence. However, it's important to note that this does not automatically result in the person’s immediate release. If a person is determined to be not guilty on the basis of insanity, but the factual aspects of the offence have been proven, the person may be made subject to orders under the Criminal Law (Mentally Impaired Accused) Act 1996. The effect of these orders could include involuntary treatment in the community or in a psychiatric facility and prolonged detention.

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