Can Mental Illness Be Used as a Legal Defence in NSW? Understanding the M’Naghten Rule

You’ve been charged with a criminal offence, but you weren’t in a healthy state of mind. Maybe you were experiencing psychosis, mania, or severe trauma. You’re now wondering:

Can I use mental illness as a defence in court?

The short answer? Yes, but only in very specific circumstances.

At National Criminal Lawyers®, we regularly advise clients facing serious charges like assault, stalking, AVO breaches, or even homicide where mental illness plays a central role. Many people confuse a mental illness defence with a section 14 mental health application, but the two are legally very different.

In this blog, we’ll explain how the mental illness (formerly ‘insanity’) defence works in NSW — including the legal test, how it is raised, and what happens if the court accepts it.

What Is the Mental Illness Defence in NSW?

The mental illness defence (also known as the McNaghten defence) is a complete legal defence to a criminal charge. If successful, it leads to a verdict of “not guilty by reason of mental illness.”

This defence is governed by section 28 of the Mental Health and Cognitive Impairment Forensic Provisions Act 2020 (NSW).

Under this section, a person is not criminally responsible for an offence if, at the time of the conduct:

“The person had a mental health impairment or a cognitive impairment that had the effect that the person did not know the nature and quality of the act, or that it was wrong.”

This rule is based on the historic McNaghten Rule, a long-standing legal standard in common law.

What Must Be Proven?

Even though the overall matter is criminal, the mental illness defence is treated as a special defence, and the burden of proof shifts to the defendant once it is raised. The standard of proof is:

“On the balance of probabilities” — i.e. more likely than not.

So while the prosecution must prove the elements of the offence beyond reasonable doubt, the accused must prove the defence of mental illness on the balance of probabilities.

To succeed with this defence, the court must be satisfied on the balance of probabilities that:

• The accused had a mental health impairment or cognitive impairment; and
• As a result, they did not understand:
  • The nature and quality of their actions; or
  • That what they were doing was wrong (in a moral and social, not just legal, sense)

This is not about whether someone has a diagnosis — it’s about whether that condition severely impacted their awareness during the offence.

Examples of When the Defence May Apply

• A person with schizophrenia who assaults someone due to paranoid delusions.
  
• A person in a manic episode who commits arson without understanding the consequences.
  
• A person with intellectual disability who does not understand that taking an item is stealing.

Each case depends on the mental state at the time of the offence, not merely the presence of a diagnosis.

How Is the Defence Raised?

• It can be raised by the defence, prosecution, or the court itself.
  
• It must be supported by psychiatric evidence, usually from a forensic psychiatrist.
  
• If accepted, the court enters a finding of “not guilty by reason of mental illness”, and refers the person to the Mental Health Review Tribunal (MHRT).
  

What Happens After a Not Guilty Verdict?

A person found not guilty by reason of mental illness does not go to prison, but also does not walk free.

Instead, their case is managed by the MHRT, which may order:

• Detention in a mental health facility, or
  
• Release into the community under supervision, with regular reviews.

Supervision can last indefinitely until the person is deemed no longer a risk to themselves or others.

Mental Illness Defence vs. Section 14 Mental Health Application

Many people confuse the two. Here’s how they differ:

	Mental Illness Defence	Section 14 Application
Purpose	Legal defence that results in not guilty	Diversion to treatment without a finding of guilt – no conviction BUT conditional dismissal
Usage	Used for serious offences (e.g. murder, assault)	Used in Local Court for less serious matters
Focus	Focuses on mental state at the time of the offence	Focuses on treatment and rehabilitation needs
Result	Leads to possible MHRT supervision	Leads to discharge with conditions

Real Case Example: R v Porter [1933] HCA 1

In this historic High Court case, a man was charged with murdering his son. The court found he was experiencing such severe mental illness that he could not understand the nature of his act.

He was found not guilty by reason of insanity (now known as mental illness), illustrating that even in very serious crimes, the defence is available.

Other notable examples include:

• R v Falconer (1990) — dissociation and trauma raised as a defence
  
  
• Adam C (NSW) — psychotic episode used in defence to murder
  
  

When Does It Arise in AVOs, Assault, or DV Cases?

This defence may apply where a person:

• Breaches an AVO during a delusional or psychotic episode
  
• Engages in stalking or threatening behaviour while severely unwell
  
• Lacks capacity to understand sexual consent or legal boundaries
  

Courts must assess not only what happened — but whether the person understood what they were doing and that it was wrong.

What Should You Do If You Think This Defence Applies?

• Don’t assume your condition will be obvious to the court — it must be proven with evidence
  
• Contact a criminal defence lawyer immediately
  
• Gather existing psychiatric history, treatment records, or hospital reports
  
• Arrange an independent forensic psychiatric assessment
  

At National Criminal Lawyers®, we:

• Assess whether the defence applies in your situation
  
• Engage expert psychiatrists to write reports
  
• Advocate for your rights in court and before the MHRT
  
• Navigate complex matters involving serious mental illness and criminal responsibility
  

Final Thoughts: A Powerful Defence, Not a Shortcut

Mental illness is not a get-out-of-jail-free card — but when properly raised, it can prevent wrongful convictions and ensure vulnerable people are treated, not punished.

If you or someone you know was mentally unwell at the time of an alleged offence, you may have a complete legal defence — but you need to act fast, and act smart.

Contact National Criminal Lawyers® Today

If mental illness is a factor in your criminal case, get the defence you deserve.

Call us now on 1800 CRIM LAW for a free first consultation.

We offer:

• Fixed fees
  
• Defence in serious mental illness matters
  
• Psychiatric referrals and forensic strategy
  
• Representation in Sydney, Parramatta and across NSW
  

We don’t just know the law — we understand the people behind it. Let us fight for you.