Section 23A of the Crimes Act 1900 (NSW) outlines the partial defence to murder of substantial impairment of abnormality of the mind.

There are two conditions that must be satisfied for the partial defence to be made out at law, these are:

1. 1. At the time of the act(s) or omission(s) causing the death, a person’s capacity to understand events, or to judge whether the person’s actions were right or wrong, or to control himself or herself, was substantially impaired by an abnormality of the mind from an underlying condition; and
   2. The impairment was so substantial as to warrant liability for murder being reduced to manslaughter

The onus falls on the person accused to prove that he or she shall not be convicted of murder because they were substantially impaired by an abnormality of the mind.

On 27 March 2021, the Mental Health Cognitive Impairment Forensic Provisions Act 2020 (NSW) amended s. 23C of the Crimes Act 1900 (NSW) to outline when a person has a cognitive impairment.

The additional provisions state that a person has a cognitive impairment if:

1. The person has an ongoing impairment in adaptive functioning, and
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 (9) or for other reasons.

Subsection 23C(9) states that:

A cognitive impairment may arise from any of the following conditions but may also arise for other reasons:

1. intellectual disability
2. borderline intellectual functioning
3. dementia
4. an acquired brain injury
5. drug or alcohol related brain damage, including foetal alcohol spectrum disorder
6. autism spectrum disorder.