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Partial Defences To Murder

Partial defences to murder are different to complete defences and work to reduce the criminal culpability of the accused from murder down to manslaughter. This article outlines the partial defences to murder that are available in NSW.

In NSW there are several partial defences available to the charge of murder:

  1. Extreme provocation;
  2. Excessive self-defence;
  3. Substantial impairment because of mental health impairment or cognitive impairment;
  4. Infanticide

The operation of these partial defences allow for an accused who would otherwise be found guilty of murder, to instead be convicted of the lesser charge of manslaughter.

Extreme Provocation

Extreme provocation exists where the conduct of the deceased towards the accused is so severely provocative that the accused is incited to respond in a manner that caused the death of the deceased.

Under section 23(2) of the Crimes Act 1900, an act can only be said to have been done in response to extreme provocation where:

  1. The act of the accused that causes death was done in response to the conduct of the deceased towards or affecting the accused; and
  2. The conduct of the deceased was a serious indictable offence; and
  3. The conduct of the deceased caused the accused to lose self-control; and
  4. The conduct of the deceased could have caused an ordinary person to lose self-control to the extent of intending to kill or inflict grievous bodily harm on the deceased.

Extreme provocation is not available if the deceased made a non-violent sexual advance to the accused, or where the accused incited the conduct of the deceased to provide an excuse to use violence against the deceased (for example, baited him into a fight). Additionally, evidence of self-induced intoxication cannot be taken into account when considering whether the accused acted in response to extreme provocation.

It is important to note that loss of self-control by the accused is subject to an objective test of the ‘ordinary person’. In applying the objective test, the actions of the ‘ordinary person’ to be considered are that the ordinary person is both sober and of the same relative age and maturity as the accused.

There is also no requirement that the death must follow the provocative conduct immediately. Whilst there must be a connection between the conduct of the deceased and the accused’s loss of self-control, the loss of self-control can develop after a prolonged period of abuse, without the need for a specific ‘triggering’ incident. One such example is known as ‘battered wife syndrome’ which may arise where a partner or wife has been subjected to years of serious physical and mental abuse.

Where the accused raises extreme provocation as a defence, the prosecution must prove beyond reasonable doubt that the act causing the death was not done in response to extreme provocation by the deceased.

Excessive self-defence

A person who acts in self-defence when committing a crime may have a full defence. Where they act in ‘excessive’ self-defence, it is a partial defence.

Self-defence is a defence that renders otherwise unlawful conduct lawful. For example, if you are in an argument with someone and during the argument they pick up a knife and you believe they are about to stab you, you may punch them several times to protect yourself. However, self-defence is limited in its application as it requires the conduct of the accused to be reasonable in the circumstances as they perceive them.

Section 421 of the Crimes Act 1900 provides for the partial defence of excessive self-defence and is available in circumstances where the accused uses force that results in the death of the deceased, and the accused believes the conduct is necessary to:

  1. Protect themselves or another; or
  2. Prevent/terminate their unlawful deprivation, or the unlawful deprivation of another; but

The conduct is not reasonable in the circumstances perceived by them.

Substantial mental health or cognitive impairment

Substantial impairment is a defence that operates where the accused was so affected by a mental health condition or cognitive impairment that they could not appreciate the nature of their actions. Substantial impairment can provide a full defence to absolve the accused of criminal responsibility except where the accused is charged with murder, in which case substantial impairment can only act as a partial defence.

Section 23A of the Crimes Act 1900 outlines the partial defence of substantial impairment because of mental health impairment or cognitive impairment. Thi ssection was amended with the introduction of the Mental Health and Cognitive Impairment Forensic Provisions Act 2021 introduced on 27 March 2021 and replaces the previous partial defence of ‘diminished responsibility’, which was repealed on 3 April 1998.

In considering the partial defence of substantial impairment, section 23A(1) of the Crimes Act 1900 requires that accused must prove on the balance of probabilities (that it was more probable than not) that:

  1. At the time of the acts causing death, the accused’s capacity to:
    1. Understand events; or
    2. To judge whether their actions were right or wrong; or
  2. To control themselves;

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

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

Section 4 of the Mental Health and Cognitive Impairment Forensics Provisions Act 2021 defines the test to be satisfied for a finding of mental health impairment and can include conditions such as anxiety disorder, an affective disorder (including clinical depression and bipolar disorder) or a psychotic disorder. However, it does not include circumstances where the impairment results solely from the temporary effect of the accused digesting a substance, or in circumstances where the impairment is caused solely by a substance abuse disorder.

Cognitive impairment is defined under section 5 of the Mental Health and Cognitive Impairment Forensics Provisions Act 2021 and can include intellectual disability, borderline intellectual functioning, dementia, or an acquired brain injury.

Section 151 of the Criminal Procedure Act also requires that the accused give notice of an intention to lead evidence of substantial impairment at trial.


Section 22A of the Crimes Act 1900 governs the partial defence of infanticide. This defence can be relied on where a woman on trial for the murder of her child (aged under 12 months) willfully caused the death of her child but at the time of the act, the balance of her mind was disturbed because she had not fully recovered from the effect of giving birth or by reason of the effect of lactation consequent upon the birth of the child. Where this is proven, the woman may be found not guilty of murder and instead guilty of infanticide and sentenced as if she were guilty of manslaughter.

If you require legal advice or representation in any legal matter, please contact Armstrong Legal.

Cara Maynard - Senior Associate – Canberra

This article was written by Cara Maynard - Senior Associate – Canberra

Before joining the team at Armstrong Legal, Cara worked as a DNA expert preparing and giving DNA evidence in criminal trials. In this role she liaised with police, DPP and defence practitioner regarding a variety of matters including DNA transfer, deposition and recovery. Cara has reviewed and interpreted thousands of DNA profiles that were reported as intelligence to the NSW...

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