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The defence of substantial impairment is contained in section 23A of the Crimes Act 1900 (NSW) which provides that a person who would otherwise be guilty of murder may instead be found guilty of manslaughter if the act or omission that constituted the killing occurred because, at the time of the conduct, the defendant’s capacity to understand events, or to judge whether his or her actions were right or wrong, or to control himself or herself, were substantially impaired by a mental health impairment or cognitive impairment.

The defendant has a ‘cognitive impairment’ for the purposes of the section if:

  1. He or she has an ongoing impairment in adaptive functioning,


As well as in comprehension, reason, judgment, learning or memory, and

  1. Those impairments result from damage to or dysfunction, developmental delay or deterioration of the brain or mind arising from:

  2. Intellectual disability;

  3. Borderline intellectual functioning;

  4. Dementia;

  5. Acquired brain injury;

  6. Drug or alcohol related brain damage, including foetal alcohol spectrum disorder; or

  7. Autism spectrum disorder.


The defendant has a ‘mental health impairment’ if:

  1. He or she has a disturbance of thought, mood, volition, perception or memory;

  2. His or her disturbance is significant for clinical diagnostic purposes; and

  3. His or her disturbance impairs emotional wellbeing, judgment or behaviour.


The effects of any self-induced intoxication are to be disregarded when determining whether the defendant has a cognitive impairment or mental health impairment.

The onus is on the defendant to establish ‘on the balance of probabilities’ that he or she has a cognitive impairment or mental health impairment for the purposes of the defence.

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