Defence of Mental Impairment

Updated: Sep 21

Fundamental to our sentencing laws is punishment for criminal deeds committed intentionally, recklessly or negligently by an offender (the guilty mind).


Mental impairment provides a defence to the actions of an offender that would otherwise merit punishment under our criminal justice system. This situation arises where an accused is said to have a mental impairment, thereby negativing the element of conscious and deliberate intent that the prosecution is required to prove.


Definition of ‘Mental Impairment’ derives from Common Law


The Act does not define ‘mental impairment’, but the term has been judicially considered in several cases R v. Sebalj [2003] VSC 181; R v. Gemmil (2004) 8 VR 242; DPP v. Taleski [2007] VSC 183 and was in these cases held to be synonymous with the definition that applied to the previous (and abrogated) common law defence of insanity, namely “a disease of the mind”.



Presumption that a person not suffering from mental impairment



Before a defence of mental impairment is established there must be evidence that negatives the presumption that an accused was not suffering from a mental impairment until evidence establishes the contrary (section 21(1) of the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997) (The Act)


The position is the same at common law (Sodeman v. The Queen (1936) 55 CLR 192; R v. Porter (1933) 55 CLR 182


When the defence of Mental Impairment Arises at Law


The defence of mental impairment is raised in two situations, as contained under section 20(1) Of the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997 and derives from the common law test:


  • An accused did not know the nature and quality of what they were doing; or

  • An accused did not know that their conduct was wrong


How the defence of Mental Impairment is raised


The defence of mental impairment is typically raised through expert opinion and the finding that an accused person was mentally impaired at the relevant time is one that is made on the balance of probabilities.


Importantly the defence only applies to an accused person who was suffering from a mental impairment at the time they committed the criminal act.


Mental Impairment and Fitness to Trial under the Act


This is to be differentiated from the position where an accused is unfit to stand trial in addition to suffering a mental impairment at the relevant time of the offence (see Part 2 of the Act – Unfitness for Trial).


Unfitness for Trial considerations under Section 6 of the Act


Section 6 of the Act sets out several factors relevant to the issue of whether an accused is fit for trial:


  • The accused is unable to understand the nature of the charge; or

  • Unable to enter a plea to the charge and to exercise the right to challenge jurors or the jury; or

  • Unable to understand the nature of the trial (namely that it is an inquiry as to whether the person committed the offence); or

  • Unable to follow the course of the trial; or

  • Unable to understand the substantial effect of any evidence that may be given in support of the prosecution; or

  • Unable to understand the substantial effect of evidence that may be given in support of the prosecution; or

  • Unable to give instructions to his or her legal practitioner

Effect of Successful Defence


The effect of successfully arguing the defence of mental impairment is that an accused will be found not guilty of the alleged offence.


Under section 23 of the Act, a finding of not guilty because of mental impairment will require the court to:


(a) Declare that the person is liable to supervision under Part 5; or


(b) Order the person to be released unconditionally


Supervision Orders under the Act


Under section 26 of the Act, a supervision order made pursuant to section 23(1)(a) may involve a person being:


(a) Commit a person to receive supervision in prison or another appropriate place (custodial supervision order); or

(b) Release the person on conditions decided by the Court and specified in an Order (non-custodial supervision order)


Before committing a person to custody under a custodial supervision order a Court must receive it has received a certificate stating that the facilities or services necessary for the Order are available (section 26(3). A further condition to the imposition of a supervision Order is that the Court must be satisfied that there are no practical alternatives in the circumstances.


Further reading


Judicial College of Victoria – Mental Impairment




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