How Does the Defence of Mental Impairment Work in Australian Capital Territory Law?

I was insane, Your Honour! How does the defence of mental impairment work in Australian Capital Territory law?

Mental impairment is one of the most talked about defences that criminal defendants raise in a court of law. Sometimes, the defence of mental impairment and the way it is perceived in “getting someone off” a crime means the defence makes its way into the media. People tend to think “I’ll just say that I was nuts, and that’ll get me off”.

However, a mental impairment defence is complicated, and just because you are found not guilty by reason of mental impairment, it does not mean the matter ends there.

In this article, we discuss how mental impairment works as a defence, how it works at sentencing if you raise mental impairment, and why the defence is so complicated.

Mental Impairment vs Fitness to Plead

The defence of mental impairment and the question of unfitness to plead are often raised in the same case, but they are distinct legal concepts.

Mental impairment is about whether the accused person committed and understood their actions at the time of the offence. This defence focuses on the mental state of the accused person when the alleged conduct occurred.

Fitness to plead, on the other hand, considers whether the accused person is capable of understanding and participating in court proceedings. It relates to their mental state at the time of the trial, not at the time of the offence.

These separate issues are evaluated through different tests, and each has its own legal implications.

Mental Impairment as a defence to a crime.

Section 28 of the Criminal Code 2002 provides that a person is not criminally responsible for an offence if, when carrying out the conduct required for the offence, the person was suffering from a mental impairment that had the effect that:

1. The person did not know the nature and quality of the conduct; or
2. The person did not know that the conduct was wrong; or
3. The person could not control the conduct.

When considering whether the person knew that the conduct was wrong, Section 28(2) provides that a person does not know that conduct is wrong if the person cannot reason with a moderate degree of sense and composure about whether the conduct, as seen by a reasonable person, is wrong.

When the person raises a defence of mental impairment, they are presumed not to have been suffering from a mental impairment. That presumption of sanity is displaced only if it is proved on the balance of probabilities that the person was suffering from a mental impairment.

So, what is a “mental impairment”?

Section 27 of the Criminal Code 2002 provides that mental impairment includes “senility, intellectual disability, mental illness, brain damage and severe personality disorder”.

Mental illness is defined as an underlying pathological infirmity of the mind, whether of long or short duration and whether permanent or temporary, but does not include a condition (“a reactive condition”) resulting from the reaction of a healthy mind to extraordinary external stimuli. A reactive condition may be evidence of mental health illness however if it involves some abnormality and is prone to recur.

All of that being said, having a mental health illness or brain damage alone is not enough to absolve someone of criminal responsibility. The mental impairment must have the effect that either the person does not know that their conduct was wrong, did not know the nature and quality of their actions, or that they could not control their conduct due to mental impairment.

So, for example, just because you have been diagnosed with PTSD, cognitive impairment, or another mental health illness, it does not mean you are exempt from criminal responsibility unless the illness renders you incapable of knowing that your conduct was wrong, understanding the nature and quality of your actions, or controlling your conduct.

Proving mental impairment

The court presumes every person is of sound mind unless proven otherwise. To succeed with this defence, it must be shown on the balance of probabilities that the accused person was suffering from a mental impairment that had one of the effects described under Section 28.

Expert evidence from psychiatrists and psychologists often plays a critical role in these cases. While expert witnesses can provide opinions about the accused's mental state at the time of the offence, the court considers all the circumstances, not just expert testimony, when determining whether the defence of mental impairment applies.

It’s not all roses.

Even if you succeed in establishing that you are not guilty due to mental impairment, that may not be the end of it. If your case is before the ACT Supreme Court, Section 28 provides that the Court must return a special verdict that you are not guilty of mental impairment, which, for example, could result in you spending that amount of time you might have spent imprisoned in a secure mental health unit. You may otherwise be subjected to mental health orders or forensic mental health orders in the ACT Civil and Administrative Tribunal. 

As such, the perception that it’s a “get out of jail free card” is misguided. Sometimes, mental impairment can result in a person being locked away, but in a different way.

The Role of ACAT in Mental Impairment Cases

If a person is found not guilty due to mental impairment, the matter does not end with the verdict. Instead, it is referred to the ACT Civil and Administrative Tribunal (ACAT) under the Mental Health Act for further assessment.

ACAT can make one of the following orders:

1. Psychiatric Treatment Order (PTO): Authorises involuntary treatment in a mental health facility if the person has a mental health illness and poses a serious risk to themselves or others. It may also include restrictions on communication.
2. Community Care Order (CCO): Allows for treatment, care, and support either in the community or a mental health facility, with possible restrictions on freedom, where the person has a mental disorder and poses significant risks.

These orders involve detailed consultations before being made, as they impose restrictive and involuntary measures.

How Mental Impairment Can Affect Sentencing

If you don’t raise it at your trial but raise it at your sentence and you have evidence to support it, mental impairment can have an impact on your sentence and potentially reduce it.

Due to the decision of R v Verdins (2007) 16 VR 269, mental impairment can affect sentencing in a few different ways:

• Reduce moral culpability, thus affecting just punishment in regard to denunciation
• Influence the sentence or the conditions in which it could be served
• Reduce the effectivity of a particular sentence, depending on the gravity of the accused’s situation
• Increase hardships of the accused upon being imprisoned due to the mental illness
• Justify the lessening of the gravity of a sentence due to the risk that imprisonment may pose upon the accused

Experienced criminal lawyers may raise these principles at a sentencing hearing, and they are colloquially referred to by lawyers as the Verdins principles.

Mental Impairment in Other Australian States & Territories

In New South Wales (NSW), significant reforms have updated the way mental health conditions are addressed in the criminal justice system. The Mental Health and Cognitive Impairment Forensic Provisions Act 2020 (NSW) replaced the older Mental Health (Forensic Provisions) Act 1990 (NSW). Under the old framework, Section 38 required a jury to find a defendant not guilty by reason of mental illness if evidence demonstrated they were suffering from a mental health illness that rendered them not responsible for their actions at the time of the offence. This defence was rooted in the historic M’Naghten rules, which defined insanity as a defect of reason, from disease of the mind, as not to know the nature and quality of the act [the defendant] was doing; or, if he did know it, that he did not know he was doing what was wrong.

The new legislation replaces the terminology of "mental illness" with a more inclusive test for mental health impairment or cognitive impairment. Under the statutory test in the 2020 Act, a person who had a mental health impairment, a cognitive impairment, or both at the time of committing the act will not be criminally responsible if:

• The mental health or cognitive impairment prevented them from knowing the nature and quality of the act; or
• They did not know the act was wrong because they could not reason with a moderate degree of sense and composure about whether the act was wrong.

This shift from "mental health illness" to recognising cognitive impairment reflects a broader understanding of conditions such as intellectual disabilities, brain injuries, and neurodevelopmental disorders. These reforms also replace the old verdict with a special finding of act proven but not criminally responsible. The new framework permits the court to enter this special verdict at any stage of proceedings if both the defendant and the prosecution agree.

In Victoria, the approach to mental and cognitive impairment remains governed by the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997 (Vic), which similarly acknowledges both mental health illness and cognitive impairment. Like NSW, the Victorian legislation continues to focus on whether a person’s mental state — due to a disease of the mind or other impairments — prevented them from understanding the nature or wrongness of their actions.

Courts in both NSW and Victoria heavily rely on expert testimony to determine whether a disease of the mind or cognitive impairment affected a person’s criminal responsibility. These frameworks underscore the balance between holding individuals accountable and ensuring those with genuine impairments are treated justly within the legal system.

The Bottom Line

Mental impairment may have a major impact on whether you can be acquitted of a criminal offence or have your sentence reduced due to your mental illness being taken into account. If you have a mental illness, you absolutely should disclose to a criminal lawyer your specific mental disorders, as this may greatly assist you in your case.

Are you looking for a knowledgeable criminal lawyer to represent you in Canberra, Queanbeyan, Wollongong, Goulburn and surrounds? Get in touch with Andrew Byrnes Law Group today and have one of our lawyers find the best possible defence for your criminal matter.