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Section 23A of the Crimes Act 1900 provides a partial defence to murder of substantial impairment by abnormality of mind (which was previously recognised in NSW as diminished responsibility). The section restricts interpretation of the phrase “abnormality of mind” to mean the accused’s capacity to:

  1. understand events, or

  2. judge whether his or her actions were right or wrong, or

  3. control himself or herself.

To prove that you suffered from an abnormality of the mind at the time of the offence, it may be necessary to obtain expert medical evidence of your underlying mental or cognitive condition.


As there are varying degrees of impairments affecting thinking and judgment, the defence is not available in discrete or minor situations to reduce the moral culpability of a person who has murdered another. Section 23A(1)(b) expressly requires that the impairment must have been so substantial as to warrant liability for murder being reduced to manslaughter. Self-induced intoxication cannot be taken into account.


In order to raise the partial defence of “substantial impairment of the mind” you must demonstrate the following:

1. at the time of the act causing death, your capacity either to:

(i) understand events, or

(ii) judge whether your actions were right or wrong, or

(iii) control yourself,

was substantially impaired by an abnormality of mind arising from an underlying condition, and

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


If you or someone else you know has been charged with murder and they think they may have the partial defence of substantial impairment of the mind available to them, it is important to obtain legal advice. Should you wish to discuss your options in more detail, please contact KF Lawyers to arrange for a free initial consultation.

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