Section 23A Crimes Act 1900 provides a partial defence to murder of substantial impairment by abnormality of mind. The current form of the section applies to any charge of murder alleged to have been committed on or after 3 April 1998. It replaced the partial defence of “diminished responsibility” (see [6-100]). Section 23A makes explicit in the opening words of subs (1) that “substantial impairment by abnormality of mind” only arises where all other issues on a charge of murder, including self-defence and provocation, have been resolved in favour of the Crown.
The provision restricts the phrase “abnormality of mind” to the accused’s capacity to:
understand events, or
judge whether his or her actions were right or wrong, or
control himself or herself.
As to the aetiology of “abnormality of mind”, the provision requires only that it should arise from an “underlying condition” which is exclusively defined as “a pre-existing mental or physiological condition other than a condition of a transitory kind”.
Section 23A(3) provides that the effects of “self-induced intoxication”, as defined in s 428A Crimes Act 1900, are to be disregarded for the purpose of determining whether the accused, by reason of this section, is not liable to be convicted of murder. R v Gosling  NSWCCA 351 at  and Zaro v R  NSWCCA 219 at – are examples where a judge was required to give a direction that self-induced intoxication at the time of the offence was to be disregarded by the jury.
It is not enough that the accused suffers from a “substantial impairment by an abnormality of mind”. Section 23A(1)(b) expressly requires that the impairment must have been so substantial as to warrant liability for murder being reduced to manslaughter. Section 23A(2) provides that opinion evidence is inadmissible on this issue.
The onus and burden of proof is upon the accused in both provisions, and in both cases the Crown is entitled to raise mental illness if the accused raises diminished responsibility, to be proved on the balance of probabilities, and vice versa, see: R v Ayoub  2 NSWLR 511. Section 151 Criminal Procedure Act 1986 requires notice to be given of the intention to raise a defence of substantial impairment and also deals with the stage at which evidence in rebuttal may be given in the Crown’s case. Section 151(3) provides:
Any evidence tendered to disprove a contention of substantial mental impairment may, subject to any direction of the court, be given before or after evidence is given to prove that contention.
The current form of s 23A largely reflects recommendations by the New South Wales Law Reform Commission, Partial Defences to Murder: Diminished Responsibility, Report No 82, 1997.
The partial defence of “substantial impairment by abnormality of mind” will succeed if [the accused] has established more probably than not, both that:
at the time of the act causing death, [his/her] capacity either to:
understand events, or
judge whether [his/her] actions were right or wrong, or
was substantially impaired by an abnormality of mind arising from an underlying condition, and
the impairment was so substantial as to warrant [his/her] liability for murder being reduced to manslaughter.
As to question no 1 above, the issue is whether [the accused’s] capacity to function in one or other of the three ways ((i)–(iii)) was substantially impaired, not whether [he/she] simply chose not to function in that way.
An “abnormality of mind” occurs if [the accused’s] capacity to function in any one of the three ways mentioned so differed from that of ordinary human beings that the reasonable person would term it abnormal.
An “underlying condition” is a pre-existing mental or physiological condition. It does not have to be a permanent condition. It is not enough if it is a passing condition or one that only lasts for a short time.
As to question no 2 above, in determining whether an impairment was so substantial as to warrant liability for murder being reduced to manslaughter. This is a value judgment applying community standards.
As to question nos 1 and 2 above, the effect of self-induced intoxication from drugs and/or alcohol at the time of the act(s) causing the death are to be disregarded.
If [the accused] has not established the partial defence of substantial impairment by abnormality of mind, the appropriate verdict is one of “guilty of murder”.
If [the accused] has established the partial defence of substantial impairment by abnormality of mind, you must find [him/her] “not guilty of murder” but “guilty of manslaughter”.
I next come to what has been shortly referred to during the trial as “impaired responsibility”. This only arises for your consideration if you are satisfied beyond reasonable doubt that the Crown has established all of the essential ingredients of the crime of murder [including self defence and/or that [the accused] was not acting under provocation]. If it has done so, you next have to come to this question of impaired responsibility.
The law provides that a person who would otherwise be guilty of murder is not to be convicted of that offence, but is to be convicted of the offence of manslaughter, if (at the time of the act causing the death concerned) [his/her] capacity either to understand events, or to judge whether [his/her] actions were right or wrong, or to control [himself/herself], was substantially impaired by an “abnormality of mind” arising from an underlying condition; and furthermore, that that impairment was so substantial as to warrant liability for murder being reduced to manslaughter.
It is called a “defence to murder” because it is for [the accused] to raise it and to prove it. To do so, however, [he/she] is not put to the strict standard of proof beyond reasonable doubt which is generally required of the Crown. The standard of proof on [the accused] is on the balance of probabilities. This means that if, at the end of your deliberations, you are of the view that it is more likely than not that what [the accused] claims in respect of this defence is so, then [he/she] has succeeded. It is called a “partial defence” because, if it does succeed, then the appropriate verdict is “not guilty of murder but guilty of manslaughter”.
Before turning to a more detailed discussion of the ingredients of this partial defence, I should first briefly explain the reasons why Parliament has provided for it. Persons charged with committing a crime, if convicted, are to be punished for it. One of the most important factors in determining what punishment should be awarded for the crime of which he or she is convicted is what, if any, matters in mitigation appear on the evidence which would serve to reduce the extent of the blame which should attach to that crime.
Although both involve the death of a human being, the crime of murder is, of course, a more serious crime than the crime of manslaughter, and hence manslaughter is punished less severely than murder. This is so for a number of reasons, one of which is that the culpability of a person who commits the crime of manslaughter is less than that of a person who commits the crime of murder.
A person who, from an abnormality of mind has an impaired capacity either to understand events; to judge whether his or her actions were right or wrong; or to control himself or herself, is less responsible, according to the standards prevailing in our community, than a person who has full capacity in those respects.
With reference to the capacity to understand events, it is important that you should consider [the accused’s] perception of events. These include [his/her] perception of physical acts and matters, the surrounding circumstances, what [he/she] was doing and its effects.
Accordingly, Parliament has provided for this defence which requires not only that [the accused] prove that [his/her] capacity was so impaired, but also requires that you, as the jury representing the community and applying the standards which you regard as current in the community, are satisfied that the impairment was so substantial that the liability of the accused to punishment should be reduced from that which would follow from a conviction of murder, to that which would follow from a conviction of manslaughter. Because the onus is on [the accused] in respect of these matters, as an exception to the general rule that the onus of proof is on the Crown, the standard of proof is the lesser standard of “on the balance of probabilities” rather than the higher standard of “proof beyond reasonable doubt” required of the Crown.]
Turning now to what is involved in this partial defence, the first matter which [the accused] must establish is that at the time of the … [specify act, for example, stabbing, shooting etc] causing the death charged, [his/her] capacity either to understand events, or to judge whether [his/her] actions were right or wrong or to control [himself/herself] was substantially impaired by abnormality of mind.
As to an “abnormality of mind”, it is clear, you may think, that what is “normal” as opposed to “abnormal” in these capacities differs widely in any group of people. What the law requires is that [the accused’s] capacity in any one of these respects was substantially impaired. “Impaired” has its ordinary meaning and requires proof of a capacity less or lower than the normal range. “Substantial” also has its ordinary meaning of being of substance and not slight or insignificant.
The law further requires that the abnormality of mind must come from a pre-existing mental or physiological condition which, on the one hand, is not of a passing or brief kind, but on the other is not necessarily permanent. Although the words, “mental or physiological” include conditions which have an identifiable physical origin. It also includes those which can not be traced to any identifiable physical cause but are functional in their nature, rather than organic.
In determining whether [the accused] has established that it is more likely than not that these matters were so, you will pay close attention to the evidence of the psychiatrists (or other expert witnesses), particularly on the questions which are summarised in the first paragraph of the written directions … [see: [6-570]]. These are areas in which psychiatrists … [etc, specify] have particular expertise and experience.
You are not bound, however, to accept their evidence. You are entitled to act on other evidence in the case if you think that there is other evidence which conflicts with or undermines the basis upon which the psychiatrists expressed their opinions.
On the other hand, you would obviously pay careful and close attention to what the opinion evidence is as to these matters because of the experience and expertise which these witnesses have in this field.
You would only decline to act on the evidence of the psychiatrists [and psychologists] if you think that there is other evidence which outweighs the psychiatric evidence, or if you think that the facts differ from those on which the psychiatrists proceeded, or if you think that the reasons expressed by the psychiatrists for their opinions (even having regard to their expertise) do not support their conclusion … [a different direction would need to be given if, as often happens, the psychiatric or psychological evidence reaches different conclusions].
The next question which you must consider is one which the Parliament has provided you, and one which you alone should come to a conclusion about. This is the question whether the substantial impairment relied upon by [the accused] was so substantial as to warrant [his/her] liability for murder being reduced to manslaughter.
This will only arise for consideration if [the accused] has satisfied you as to the matters referred to in the first paragraph of the written directions which have been distributed to you.
This, as I have indicated, is a matter for you to decide, applying the standards which you regard as prevailing in our community (bearing in mind that manslaughter is regarded as a less serious crime than murder, and that the community places less blame and condemnation upon a person guilty of manslaughter than of murder).
The question you should ask yourself is — “Has [the accused] satisfied you in the circumstances of this case that any impairment to [his/her] capacity (if you find that it is likely to have existed) was such that [he/she] should not be condemned or blamed as a murderer, and that rather, [he/she] should be treated as having been guilty of manslaughter?”.
In answering this question, you should approach the matter in a broad common sense way, applying (as I have said) the standards of the community which you are here to represent. In deciding this, you may think that a substantial impairment would have to be significantly substantial so as to warrant a reduction if the blameworthiness of [the accused] to “manslaughter”. That is the question which you must decide.
To summarise then, if, on the one hand, you have been satisfied by the Crown beyond reasonable doubt of all of the necessary matters which it has to establish in order to justify a conviction of murder, and also that [the accused], on the other hand, has satisfied you that it is more likely than not that this partial defence is made out, the appropriate verdict is “not guilty of murder” but “guilty of manslaughter”.
If, however, you have been satisfied by the Crown beyond reasonable doubt of all that it must prove to justify a conviction for murder, but [the accused] has failed to satisfy you that it is more likely than not that this partial defence has been made out, the appropriate verdict is one of “guilty of murder”.