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Full Defences to Murder

A full defence to murder is a defence that absolves the defendant of criminal responsibility completely. There are also partial defences to murder, which reduce the verdict to the less serious charge of manslaughter. This article outlines the full defences to murder that exist in New South Wales.

Reasonable doubt

A person will not be found guilty of a criminal offence if there is reasonable doubt that they committed the offence. The elements of murder are:

  1. The victim died;
  2. The act or omission of the accused caused the death of the victim;
  3. The act of the accused was without lawful cause or excuse; and
  4. The act or omission causing death was done by the accused with the intention to kill or do grievous bodily harm, foreseeing that it was probable that the death of a person would result from the act or omission; or in an attempt by the accused or an accomplice to commit a serious indictable offence punishable by imprisonment for life or 25 years

If the prosecution fails to prove any of the above elements beyond a reasonable doubt, the accused must be acquitted (found not guilty) of murder.


Self-defence is a full defence to murder. If a person kills another person in self-defence or in defence of another person, they are not guilty of an offence. Rather, they have committed justifiable homicide.

Section 418 of the Crimes Act outlines where the defence is available and the requisite elements were discussed in the case of R v Katarzynski [2002].

  • 1. Subjective element
  • The accused acts in self-defence if and only if there is a reasonable possibility that the accused genuinely believed that his or her conduct was necessary:
  • (a) to defend him/herself or another person, or
  • (b) to prevent or terminate the unlawful deprivation of his/her/another’s liberty, or
  • (c) to protect property from unlawful taking, destruction, damage or interference, or
  • (d) to prevent criminal trespass to any land or premises or to remove a person committing any such criminal trespass.
  • In considering whether the accused held a genuine belief, all of the accused’s personal characteristics at the time of the conduct in question will be considered.
  • Importantly, s 420 of the Crimes Act states that (c) and (d) above cannot be used to establish the defence where the accused uses force that involves the intentional or reckless infliction of death.
  • 2. Objective element
  • It must be determined whether the accused’s response was reasonable in the circumstances as he or she perceived them. In this aspect, the jury decides what matters it should take into account, but it may at least have regard to the accused’s age, gender and health.
  • Where the accused’s conduct involved the infliction of death and was not a reasonable response in the circumstances but the accused believed the conduct necessary to defend himself, herself or another person and/or to prevent the unlawful deprivation of his, her or another’s liberty, the accused may be found guilty of manslaughter.

The defendant will have an ‘evidentiary burden’ such that once the defence is raised, the Crown must prove beyond reasonable doubt that the accused did not act in self-defence. To do so, the Crown must show that either:

  • The accused did not genuinely believe that it was necessary to act as he or she did in his or her own defence; or
  • What the accused did was not a reasonable response to the danger, as he or she perceived it to be.


A key element that the prosecution must prove is that the accused performed the acts constituting the offence “voluntarily”, that the acts in question were willed bodily movements. Therefore, the accused is not criminally responsible for acts committed while in a state of automatism. Examples of states of automatism include: sleepwalking, suffering concussion, under anaesthetic, suffering from some forms of epilepsy, an uncontrolled act such as sneezing. It does not include self-induced intoxication.

Automatism principles

It is presumed that the accused’s acts were voluntary. The accused has an ‘evidentiary burden’ to show that there is a reasonable possibility that his or her acts were not voluntary. The Crown must then prove beyond reasonable doubt that the relevant acts were voluntary. If the Crown cannot prove this beyond reasonable doubt, the accused will be acquitted.

This defence does not include the situation where an underlying mental illness which is prone to reoccur deprives the accused of the capacity to control his or her actions. Such cases may fall within the defence of substantial impairment or the defence of mental illness, depending on the cause of the accused’s condition at the time of the acts.


The accused will be acquitted where he or she acted because of threats (express or implied) of death or really serious injury to himself, herself or dependants, and a person of ordinary firmness and strength of will in the accused’s position would have yielded to those threats.

In New South Wales, the defence of duress is not available to a principal in the first degree of murder (where the accused did the killing). It is, however, available where the accused did not do the killing, but was charged as an accessory.


Necessity may be available where circumstances bear upon the accused, inducing him or her to break the law to avoid even more dire consequences.

The courts have not clearly decided whether the defence of necessity is available to the principal in the first degree for murder.

The leading New South Wales case on the defence is R v Rogers, which requires the following elements to be shown in order to establish the defence:

  1. Irreparable evil.

The accused must have acted only in order to avoid certain consequences (that is, they were bound to happen and could not otherwise be avoided) which would have inflicted death or serious injury (“irreparable evil”) upon the accused or others whom he or she was bound to protect.

  1. Imminent peril.

The accused must honestly believe on reasonable grounds that he or she was placed in a situation of imminent peril. Proportion.

The accused’s acts to avoid the imminent peril must not be out of proportion to the peril being avoided. An instructive question to ask is whether a reasonable person in the accused’s position would have considered what he did necessary to avoid the peril? Were there reasonable alternatives open to the accused that did not involve breaking the law, such as bringing the threat to the attention of an authority or seeking protection?

Mental impairment

The defence of mental impairment is provided for under the Mental Health (Forensic Provisions) Act 1990. If the court is satisfied that at the time of the offence the accused was so mentally impaired that they did not understand the nature of the act that forms the basis of the charge, the law will not hold them criminally liable. The jury will return a special verdict of “not guilty by reason of mental impairment”.

It is presumed that the accused was of sound mind when he or she committed the offence in question. The accused must prove on the balance of probabilities (more probable than not) that:

  • 1. At the time of the act, the accused was labouring under such a defect of reason that he or she could not reason with a moderate degree of sense and composure.
  • 2. The defect of reason was caused by a “disease of the mind“. There is no legal definition of “disease of the mind” but in the case of R v Porter (1936), the Court specified that the accused’s mind must have been one of “disease, disorder and disturbance” and cannot be caused by mere transitory passions, a personality disorder or impulsiveness. The major mental illnesses falling within this defence are the psychoses, such as schizophrenia and bipolar disease.
  • 3. By reason of a disease of the mind, the accused did not know the physical nature and quality of his or her act; he or she did not know that what he or she was doing was wrong with regard to everyday standards of reasonable people.

If there is a verdict of “not guilty by reason of mental impairment“, the court may order that the person be detained in such place and in such manner as the court thinks fit until released by due process of law. The person may not be released until the court is satisfied on the balance of probabilities that the safety of the person or public will not be seriously endangered by the person’s release.

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

Michelle Makela

This article was written by Michelle Makela

Michelle has over 15 years experience in the legal industry, working across commercial litigation, criminal law, family law and estate planning.  Michelle has been involved in all practice areas of the firm and in her personal practice has had experience in litigation at all levels (State and Federal Industrial Tribunals, the Supreme Court, Court of Appeal, the Federal Court, Federal...

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