Crimes Act 1900

A person who would otherwise be guilty of murder is not to be convicted of murder if—

at the time of the acts or omissions causing the death concerned, the 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 a mental health impairment or a cognitive impairment, and

the impairment was so substantial as to warrant liability for murder being reduced to manslaughter.

For the purposes of subsection (1) (b), evidence of an opinion that an impairment was so substantial as to warrant liability for murder being reduced to manslaughter is not admissible.

If a person was intoxicated at the time of the acts or omissions causing the death concerned, and the intoxication was self-induced intoxication (within the meaning of section 428A), the effects of that self-induced intoxication are to be disregarded for the purpose of determining whether the person is not liable to be convicted of murder by virtue of this section.

The onus is on the person accused to prove that he or she is not liable to be convicted of murder by virtue of this section.

A person who but for this section would be liable, whether as principal or accessory, to be convicted of murder is to be convicted of manslaughter instead.

The fact that a person is not liable to be convicted of murder in respect of a death by virtue of this section does not affect the question of whether any other person is liable to be convicted of murder in respect of that death.

If, on the trial of a person for murder, the person contends—

that the person is entitled to be acquitted on the ground that the person was not criminally responsible because of mental health impairment or cognitive impairment, or

that the person is not liable to be convicted of murder by virtue of this section,

For the purposes of this section, a person has a cognitive impairment if—

the person has an ongoing impairment in adaptive functioning, and

the person has an ongoing impairment in comprehension, reason, judgment, learning or memory, and

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.

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

intellectual disability,

borderline intellectual functioning,

dementia,

an acquired brain injury,

drug or alcohol related brain damage, including foetal alcohol spectrum disorder,

autism spectrum disorder.