Defences to criminal charges: complete and partial defences to murder: HSC Legal Studies

What this dot point is asking

The syllabus lists "defences to criminal charges: complete defences, partial defences to murder" under the criminal trial process, and asks you to "assess the use of defences to criminal charges in achieving justice". NESA's 2024 marking guidelines for the Crime extended response (justice for adult offenders) listed complete and partial defences as relevant content, and the 2025 paper tested the basic idea in multiple choice.

To answer well you need to know what each defence requires, what happens if it succeeds, who bears the burden of proof, and how NSW has reformed defences to reflect changing community values.

The answer

How defences work

A defence is raised by the accused but, for most defences, the prosecution must then disprove it beyond reasonable doubt. Defences work in one of two ways:

1. Negating an element. Automatism means there was no voluntary act (no actus reus); mistake of fact or consent can mean the required mens rea or circumstance is absent.
2. Justifying or excusing the conduct. Self-defence, duress and necessity accept that the accused did the act intentionally, but say the law should not punish them because of the circumstances.

The distinction the syllabus draws is between complete defences, which lead to acquittal (or, for mental health impairment, a special verdict), and partial defences, which apply only to murder and reduce it to manslaughter. Partial defences exist because murder carries a maximum of life imprisonment and a standard non-parole period of 20 years (25 for the murder of certain victims such as children or police), so it is important that the conviction matches the accused's blameworthiness.

Complete defences

Self-defence

Under s 418 of the Crimes Act 1900 (NSW), a person is not criminally responsible if they believed their conduct was necessary to defend themselves or another person, to prevent unlawful deprivation of liberty, or to protect property, and the conduct was a reasonable response in the circumstances as they perceived them. The test is partly subjective (what did the accused believe and perceive?) and partly objective (was the response reasonable given those perceptions?). Once raised, the prosecution must prove beyond reasonable doubt that the accused was not acting in self-defence (s 419). Self-defence is not available where the accused intentionally or recklessly caused death only to protect property (s 420).

Mental health impairment or cognitive impairment

The Mental Health and Cognitive Impairment Forensic Provisions Act 2020 (NSW), which commenced on 27 March 2021, replaced the old common law "M'Naghten" defence of mental illness. Under s 28, a person is not criminally responsible if, at the time, they had a mental health impairment or cognitive impairment (or both) that meant they did not know the nature and quality of the act or did not know that it was wrong. The defence must be proved on the balance of probabilities. If it succeeds, the verdict is "act proven but not criminally responsible" rather than "not guilty by reason of mental illness". The change, recommended by the NSW Law Reform Commission, was intended to acknowledge to victims and families that the act did occur. The person may become a forensic patient supervised by the Mental Health Review Tribunal, which can detain them in a secure facility for as long as necessary to protect the community.

Automatism

Automatism is conduct without conscious will, such as during a seizure, concussion or sleepwalking. Because there is no voluntary act, there is no actus reus. Sane automatism (from an external cause) leads to a complete acquittal; insane automatism (from an internal disease of the mind) falls under the mental health defence (R v Falconer (1990) 171 CLR 30).

Duress

Duress applies where the accused committed the offence because of threats of death or serious injury, and a person of ordinary firmness would have yielded to the threat. In R v Abusafiah (1991) 24 NSWLR 531 the NSW Court of Criminal Appeal described duress as a complete defence because the accused lost their free choice. Duress is not available for murder or attempted murder.

Necessity

Necessity excuses conduct needed to avoid imminent and serious harm, where the accused reasonably believed the danger existed, acted proportionately and had no reasonable alternative (R v Loughnan [1981] VR 443). It is rarely successful and is not a defence to murder (R v Dudley and Stephens (1884) 14 QBD 273, the lifeboat cannibalism case).

Consent

Consent is a defence to some assaults (such as contact sports), but not where serious harm is intended. For sexual offences, the question is whether the prosecution can prove absence of consent and knowledge of it. Since 1 June 2022, NSW's affirmative consent model means an accused's belief in consent is not reasonable unless they said or did something to find out whether the other person consented (Crimes Act s 61HK), which significantly narrows the old "honest belief" argument.

Mistake

An honest and reasonable mistake of fact is a defence to strict liability offences (Proudman v Dayman (1941)) and may negate intent for other offences. Ignorance of the law is not a defence.

Intoxication

Self-induced intoxication is generally not a defence. Under Part 11A of the Crimes Act, it cannot be considered for offences of basic intent, and it can be considered only on whether the accused formed a specific intent (such as the intent to kill).

Partial defences to murder

The 2014 extreme provocation reform

The old partial defence of provocation was criticised for decades as excusing violent male jealousy and "homosexual advance" killings (in Green v The Queen (1997) 191 CLR 334 the High Court held that provocation should have been left to the jury where the accused killed a man who made a non-violent sexual advance). Public anger peaked after R v Singh [2012] NSWSC 637, in which Chamanjot Singh, who cut his wife's throat, was convicted of manslaughter on the basis of provocation. A Legislative Council Select Committee on the Partial Defence of Provocation reported in 2013, and the Crimes Amendment (Provocation) Act 2014 (NSW) replaced provocation with extreme provocation. The provoking conduct must now itself be a serious indictable offence, and a non-violent sexual advance or conduct incited by the accused cannot qualify. This is a strong example of law reform reflecting changing moral and ethical standards.

Evaluating defences in achieving justice

For the accused

Defences are essential to justice because criminal punishment should follow moral fault. People who act reasonably to protect themselves, who are forced by threats, or who could not understand what they were doing should not be treated as murderers. The prosecution usually bears the burden of disproving defences, which protects the presumption of innocence.

For victims and families

Reforms have made defences more respectful of victims: extreme provocation no longer excuses violence against partners who leave, and the 2020 special verdict acknowledges that the act happened. However, families can still find partial defence verdicts, and the release of forensic patients, deeply distressing.

For society

Forensic patients are supervised and can be detained indefinitely by the Mental Health Review Tribunal, so the community is protected even without a conviction. Manslaughter convictions still carry significant sentences.

Limits

• Complexity and cost. Mental health and substantial impairment defences depend on competing psychiatric evidence and are expensive to run, which disadvantages accused people without adequate legal aid.
• Family violence. Defences were built around sudden confrontations between strangers. Women who kill abusive partners after years of coercive control may not fit the "reasonable response" or "loss of self-control" models. In Osland v The Queen (1998) 197 CLR 316 the High Court accepted evidence of "battered wife syndrome" as relevant but upheld Heather Osland's murder conviction. Reformers argue for self-defence provisions that expressly consider family violence, as some other states have adopted.
• Consistency. Jury decisions on defences are unexplained and can vary.

Judgement. Defences achieve justice to a large extent by matching liability to blameworthiness, and NSW reforms have improved fairness to victims, but their complexity and their poor fit with family violence mean justice is not equally available to all accused people.

In one sentence

Defences decide whether an accused who did the act is criminally responsible at all (complete defences such as self-defence and mental health impairment) or less responsible (partial defences to murder such as extreme provocation and substantial impairment), and NSW reforms in 2014 and 2020 have made them fairer to victims while leaving concerns about complexity and family violence.

Try this

Q1. Outline the partial defence of substantial impairment by abnormality of mind. (3 marks)

• What the marker wants. Section 23A, the three capacities, the underlying condition, the reverse burden, and the result (manslaughter).

Q2. Explain why NSW replaced the partial defence of provocation with extreme provocation. (5 marks)

• What the marker wants. Criticism of the old defence, R v Singh, the 2013 select committee, the new requirements and exclusions, and the link to changing community values.

Q3. Assess the effectiveness of defences in achieving justice for victims of domestic violence who are charged with murder. (7 marks)

• What the marker wants. Self-defence, excessive self-defence and substantial impairment applied to the family violence context, Osland, and a judgement on the limits.