By James Stevens, Director and Solicitor, Go To Court Lawyers. Last reviewed 15 April 2026.
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In the Northern Territory, the common law defence of mental impairment is codified in section 43C of the Criminal Code Act 1983. A person is not guilty of a criminal offence if the court is satisfied that they were suffering from a mental impairment at the time of the alleged offence. However, a verdict of not guilty in this situation does not mean that there will be no consequences for the person. This article outlines the defence of mental impairment as it applies in Supreme Court matters in the NT.
Jurisdiction
It is important to note that section 43C applies only to Supreme Court matters, with a separate legislative scheme under the Mental Health and Related Services Act applying to matters in the summary jurisdiction.
What is the defence of mental impairment?
Under section 43C, a person is not guilty of a criminal offence if, at the time of carrying out the conduct, they were suffering from a mental impairment that meant that they:
- did not know the nature of the conduct; or
- did not know that the conduct was wrong; or
- or were not able to control their actions.
If this defence is established the person must be found not guilty because of mental impairment.
Onus of proof
A person is presumed not to be suffering from a mental impairment. The party raising mental impairment bears the onus of rebutting this presumption.
The question of whether a person was suffering from a mental impairment is a question of fact and must be determined by the jury.
Who may raise mental impairment?
Mental impairment may be raised:
- by the defence at any point in the trial;
- by the court on application of the prosecution;
- by the court at its own initiative.
How is mental impairment assessed?
When mental impairment is raised, the issue is to be dealt with separately from the rest of the trial. The court must hear the evidence of the accused’s mental state and may require the accused to be examined by a psychiatrist or another relevantly qualified person and a report made to the court.
After the evidence has been heard and submissions have been made, the judge must direct the jury to consider:
- whether, on the balance of probabilities, the defence is established;
- whether the evidence establishes the elements of the offence the person is charged with, or an alternative offence beyond a reasonable doubt;
- whether the person is not guilty of the offence, not guilty because of mental impairment or guilty of the offence.
Plea of not guilty because of mental impairment
If the prosecution and defence agree, a person may enter a plea of not guilty because of mental impairment. The court then records this finding without the need for a trial. This approach may be appropriate in matters where the parties are in agreement about the defence and there is no possibility that the accused will be acquitted of the charge on any other basis.
Supervisions orders
When a person is found not guilty because of mental impairment, the court must make them subject to a supervision order.
A supervision order may be custodial, requiring the person to be detained in a prison or in another appropriate place, such as a mental health ward. Supervision orders may also be non-custodial, allowing the person to be released into the community but be supervised by the Health Department.
A court may only make a custodial supervision order committing a person to a correctional facility if there is no other practicable alternative.
A supervision order is in force for an indefinite term. It can be varied or revoked by a court on application by the supervised person, their next of kin, the person who has care and control of them or by the Director of Public Prosecutions.
When a court makes a supervision order against a person, they have the same appeal rights as a person who is sentenced by a court for a criminal offence.
Criticisms of the system
The NT’s system for dealing with those who are not guilty because of mental impairment has been criticised, with some arguing that such people should be dealt with outside of the criminal justice system.
Many accused persons who are found not guilty because of mental impairment in the NT are detained in prison under custodial supervision orders because of the absence of any other suitable secure facility. This can result in a person being detained for longer than they would have been if they had been found guilty and sentenced to imprisonment, as a person is only released from a custodial supervision order if the order is varied or revoked.
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