Is Having a Mental Illness a Defence?

20 July 2025 in Criminal Law | 3 min read | skip to FAQs

This article discusses whether having a mental illness can be considered a defence to criminal charges in NSW. We examine the definitions of mental illness and cognitive impairments, the criminal defence of mental health and cognitive impairment. We also discuss Section 14 orders.

Mental Health & Cognitive Impairment Defences

Mental illness as a complete defence was originally established by the 1843 case of M’Naughten, defined as ‘a defect of reason, from disease of the mind, as not to know the nature and quality of the act [the defendant] was doing; or, if he did know it, that he did not know he was doing what was wrong.’

Today, the law regarding mental illness as a defence is contained in the Mental Health and Cognitive Impairment Forensic Provisions Act 2020. This Act (which replaced the previous Mental Health (Forensic Provisions) Act 1990 (NSW)) provides that an individual is not criminally responsible for an offence if, at the time of the act, they were experiencing a mental health or cognitive impairment that impacted them enough so they did not know:

1. The nature and quality of the act; or
2. That the act was wrong (due to a lack of reason with a moderate degree of sense and composure).

If the defence has been proven on the balance of probabilities, a judge or jury must return a special verdict of “act proven but not criminally responsible.” A special verdict can be entered by a court at any time during the proceedings if agreed upon by both the defendant and the prosecutor. In many cases, particularly serious criminal matters, the court may ask for evidence supporting the defence, such as reports from psychiatrists and other medical experts.

Section 14 Orders

Separate from a full defence of mental illness, Section 14 of the Mental Health and Cognitive Impairment Forensic Provisions Act 2020 (NSW) allows a Magistrate in the Local Court to dismiss charges without a finding of guilt if the accused is suffering from a mental health condition or cognitive impairment.

To grant a Section 14 order, the court must be satisfied that:

1. The defendant was, or is, suffering from a mental health impairment or cognitive impairment, and
2. It is more appropriate to deal with the defendant in accordance with the provisions of the Mental Health and Cognitive Impairment Forensic Provisions Act 2020 (NSW) than otherwise in accordance with law, including the Crime (Sentencing Procedure) Act 1999 (NSW).

If a Section 14 application is successful, the matter is dismissed and the accused may be discharged into the care of a responsible person, placed under treatment conditions, or subject to other community-based supports. The individual may also be required to comply with a treatment plan for up to 12 months. Section 14 offers a therapeutic, non-punitive alternative that prioritises rehabilitation and ongoing mental health support.

For further information on Section 14 pleas, please see our article on Section 14 Applications

Definitions of Mental Health or Cognitive Impairment

Section 4 of the Mental Health and Cognitive Impairment Forensic Provisions Act 2020 (NSW), provides that a person has a mental health impairment if:

1. The person has a temporary or ongoing disturbance of thought, mood, volition, perception or memory;
2. The disturbance would be regarded as significant for clinical diagnostic purposes; and
3. The disturbance impairs the emotional well-being, judgment, or behaviour of the person.

A mental health impairment may arise from:

1. Anxiety disorders
2. Affective disorders (e.g. depression or bipolar)
3. Psychotic disorders (e.g. schizophrenia)
4. Substance-induced mental disorders that are not temporary

Substance abuse disorders and intoxication are not considered mental health impairments under this Mental Health and Cognitive Impairment Forensic Provisions Act 2020 (NSW).

Under section 5 of the Mental Health and Cognitive Impairment Forensic Provisions Act 2020 (NSW), a person has a cognitive impairment if:

1. The person has an ongoing impairment in adaptive functioning;
2. The person has an ongoing impairment in comprehension, reason, judgment, learning or memory; and
3. The impairments result from damage to or dysfunction, developmental delay, or deterioration of the person’s brain or mind that may arise from a condition set out in subsection (2) or for other reasons.

Cognitive impairment may arise from:

1. Intellectual disabilities
2. Borderline intellectual functioning
3. Dementia
4. Brain injuries
5. Drug or alcohol-related brain damage
6. Autism spectrum disorder

How Can BSM Sydney Lawyers assist you?

The expert criminal lawyers at Brander Smith McKnight Sydney Lawyers have extensive experience in both Section 14 Applications and assisting those with a mental illness or cognitive impairment to defend charges. Our senior criminal lawyers offer a free 20-minute, no obligation consultation.

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BSM Lawyer Sydney has offices conveniently located in Sydney CBD, Sutherland, Parramatta, Shellharbour and Wollongong.