On the night of July 20, 2019, Jessica Camilleri stabbed and decapitated her mother in their St Clair home. Known to be suffering from multiple mental illnesses, autism spectrum disorder and an intellectual disability, Jessica Camilleri avoided the charge of murder due to her mental impairment. Although found guilty on the charge of manslaughter on 10 December 2020 in the New South Wales Supreme Court, this article will explain the defence known as the ‘Substantial Impairment by Abnormality of Mind’ found in section 23A of the Crimes Act 1900 (NSW).  


In NSW, there are several forms of ‘homicide’. These can be broken down into ‘murder’ and ‘manslaughter’Murder is the act of killing another human being where there has been an intention to kill. On the other hand, the crime of ‘manslaughter’ exists where a killing has occurred, but the accused does not possess the requisite mens rea (guilty mind) to satisfy intention in order to substantiate a murder charge. In such circumstances, the killing may have occurred due to provocation, excessive self-defence or substantial impairment by abnormality of the mind.  

If a person is charged with murder, the accused may raise the partial defence known as ‘substantial impairment by abnormality of the mind’. This defence may be available to an accused if other partial defences such as provocation and self-defence are inappropriate. The impairment by abnormality of the mind defence argues that the accused could not have possessed the mens rea (criminal intent) due to a mental impairment which would have altered their state of mind to an extent that any reasonable person would view it as abnormal.  

Having understood that, you may ask the question:  

What constitutes an “abnormal mind”? 

The law accepts that a variety of factors may impede the accused’s ability to; 

  • Understand the events which have occurred; 
  • Understand whether their actions were lawful and/or morally acceptable; and 
  • Possess the ability to control their actions.  

In order to satisfy any and/or all of the abovementioned factors, a court of law might hear evidence that the accused is afflicted with an underlying mental condition which has been diagnosed (or was undiagnosed) at the time of the offending. If evidence of an underlying condition is brought before the court, it will be the role of the jury to assess whether the underlying condition has impaired the accused to an extent that they could not be criminally liable for murder. As stated by the leading case of R v Trotter (1993): 

“The impairment must be so substantial as to warrant the reduction of the crime from murder to manslaughter”   

Should the jury be satisfied by the threshold outlined by the decision in R v Trotter (1993) the partial defence raised by virtue of substantial impairment by abnormality of mind will allow the charge of murder to be withdrawn and the accused will be found guilty of the lesser charge of manslaughter.  



The court heard that Jessica Camilleri stabbed her mother over 200 times in a fit of rage which was derived from a substantial impairment that impeded her ability to control violent outbursts.  

In order to support this claim, the court heard from two forensic psychiatrists. Both of the psychiatrists provided evidence of Jessica’s mental impairment which included; multiple mental illnesses, autism spectrum disorder and an intellectual disability. The court also heard evidence of Jessica’s history of bullying, which her defence lawyers argued exacerbated her tendency to inflict targeted aggression at the slightest provocation.   

Although the Crown Prosecutor maintained that Jessica’s condition did not remove an intention to kill her mother, defence counsel argued that the impairment described by the two forensic psychiatrists extended so far as to “limit the objective seriousness of the offence”. Before the jury could reach a verdict on murder or manslaughter, the central question is whether Jessica’s abnormality impaired her ability to control her violent outbursts on the night she killed her mother. Ultimately, the jury decided that Jessica’s condition warranted the lesser charge of manslaughter having regard to the evidence submitted by the two forensic psychiatrists. 


Section 24 of the Crimes Act 1900 (NSW): 

Whosoever commits the crime of manslaughter shall be liable to imprisonment for 25 years— 

Provided that, in any case, if the Judge is of the opinion that, having regard to all the circumstances, a nominal punishment would be sufficient, the Judge may discharge the jury from giving any verdict, and such discharge shall operate as an acquittal. 

Due to be sentenced in March 2021, the fundamental consideration when sentencing an offender for manslaughter will be an assessment of the objective seriousness of the offence. In Jessica Camilleri’s case, the issue of her mental impairment will be considered as a mitigating factor when it comes to sentencing. However, it is expected that the gruesome nature of the offending will be of some impact when weighing the appropriate punishment. In addition, the Judge will be required to give consideration for public safety. The objective seriousness of the offence and the public interest will be the guiding factors when the Court passes its sentence in March.  


At National Criminal Lawyers®, our team of specialist criminal defence lawyers are equipped with expert knowledge to assist those charged with especially serious crimes. If you or someone you know is facing a charge of murder or manslaughter, contact us now to ensure your case is heard in accordance with the fair application of the law.  

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