From the moment a person with a mental illness or intellectual disability is first interviewed by police, specific procedures, laws and human rights obligations apply. The defence of mental impairment should always be considered, but important legal factors must be considered. Assistance for going to court should be planned ahead, and consideration of criminal justice diversion programs, and the specialist Assessment and Referral Court that can provide psychological assessment and referral to welfare, health, mental health, disability, housing services, and/or to drug and alcohol treatment. More sentencing options are available and supervision and leave orders for forensic patients.


Liam McAuliffe


Fitness to stand trial and the defence of mental impairment

Last updated

1 July 2022


It is presumed at law that a person appearing before a court has the capacity to enter a plea. Generally, most people with an intellectual or psychiatric disability can plead in a court, but sometimes they cannot. In this situation, it is the obligation of the party who is asserting that the relevant person is unable to enter a plea to prove this assertion, and therefore rebut the presumption (i.e. show it does not apply).

The law in relation to fitness to plead – including the common law defence of insanity, and the legal status of indefinite detention at ‘the Governor’s pleasure’ – was reformed by the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997 (Vic) (‘CMIUT Act’).

The CMIUT Act:

  • defines the criteria for determining if a person is unfit to stand trial;
  • replaces the common law defence of insanity with a statutory defence of ‘mental impairment’;
  • provides new procedures for dealing with people who are unfit to stand trial or are found not guilty because of mental impairment.

Fitness to stand trial

Under section 6 of the CMIUT Act, a person is unfit to stand trial for an offence if their mental processes are so disordered during the trial that they are unable to:

  • understand the nature of the charge;
  • enter a plea to the charge and exercise the right to challenge jurors or the jury;
  • understand the nature of the trial (i.e. that it is an enquiry into whether or not the person committed the offence);
  • follow the course of the trial;
  • understand the substantial effect of the prosecution’s evidence; or
  • give instructions to their legal practitioner.

Note that a person who is suffering from memory loss is considered fit to stand trial (s 6(2) CMIUT Act).


When a person is found unfit to plead – and the judge determines that the defendant is unlikely to become fit within the next 12 months – a court may adjourn the matter or hold a special hearing (ss 12–18 CMIUT Act) to determine whether the person would be found:

  • not guilty of the offence;
  • not guilty because of mental impairment; or
  • to have committed the offence.

Under the CMIUT Act, a finding that the person committed the offence constitutes a qualified finding of guilt only. In the Magistrates’ Court, if a person is found not guilty because of mental impairment, they must be discharged by the court.

However, there is a risk that the Office of Public Prosecutions will attempt to have the matter heard in a higher court where a custodial or non-custodial supervision order (with conditions) can be made. This is a serious risk, and there are important legal factors that must be considered in raising this defence in the higher courts.

Defence of mental impairment

Where there is a connection between the action for which the person is charged and their mental impairment, they may be eligible for a defence of mental impairment (s 20 CMIUT Act). If this defence is established, that person must be found not guilty because of mental impairment.

A defence of mental impairment requires the client’s instructions and evidence from a treating practitioner, that at the time of the offence, the person was experiencing a mental impairment that had the effect that:

  • the person did not know the nature and quality of the conduct; or
  • the person did not know that the conduct was wrong (i.e. they could not reason with a moderate degree of sense and composure about whether the conduct, as perceived by reasonable people, was wrong).

An assessment of the offender’s mental state must take place as soon as possible after the alleged crime. If a report is not obtained as soon as possible, this could jeopardise a successful defence, as a report obtained significantly later than the alleged offence may not be relevant enough for a successful defence. If a report was not obtained as soon as possible, an offender with a disability could use the Charter of Human Rights and Responsibilities Act 2006 (Vic) – the right to liberty (s 8) or security (s 22).

It is vital for a person to have proper legal advice about the defence of mental impairment, given the serious legal implications, especially if the matter is heard in the higher courts. (See Chapter 2.4: Legal services that can help.)

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