Mental Health Defence of Substantial Impairment by Abnormality of Mind in NSW


It is reported that 27-year-old Sydney woman, Jessica Camilleri has stabbed her 57-year-old mum over 100 times following an argument.

Following the stabbing, Jessica then decapitated her mother’s head from inside their St Clair house before carrying it and placing it outside onto the footpath.

The matter was heard before a jury in the Supreme Court of NSW in Sydney in early December.

The court heard, that when police first arrived at the scene, Jessica asked them if they could work “miracles” and “sew her head back on”.

When police responded, “no that’s a bit of a stretch”, she said, “so there’s nothing that can be done to bring her back?”

“I’m certain that once you lose your head, that’s it” replied senior constable Anthony D’Agostino.

The court also heard that, during a police interview on the night of the incident in July 2019, Jessica told police that she had been arguing with her mum in respect of going to a mental health facility. As her mother went to dial triple zero, Jessica smacked her mum’s phone away.

By this stage, as her mum could not locate another phone, there was a physical altercation between the two.

At court, Jessica pleaded not guilty to the charge of murder, which was on the basis of using the partial mental health defence of substantial impairment by an abnormality of the mind as a result of a loss of control.

On Thursday 10 December, as Jessica was observed increasingly anxious leading up to the jury verdict, the jury returned a verdict of ‘not guilty’ for the murder charge due to a mental impairment, but guilty of manslaughter.

She is expected to be sentenced early next year in February 2021.

During evidence, the court heard that Jessica was a fan of horror films. Police discovered 8 copies of the horror film, The Texas Chainsaw Massacre, in addition to 5 copies of Jeepers Creepers, in her home.

Murder and manslaughter offences in NSW carry heavy terms of imprisonment. Murder carries up to life imprisonment, while manslaughter carries up to 25-years imprisonment.

Defence of Substantial Impairment by Abnormality of Mind in NSW

The partial defence to a murder charge of substantial impairment by abnormality of mind is expressed in section 23A of the Crimes Act 1900 (NSW).

The partial defence applies for murder alleged to have been committed on 3 April 1998 or after. In addition, it only applies if any other issues of self-defence and provocation do not apply in favour of the defence.

If a person is charged with murder, he/she may raise and argue the partial defence of substantial impairment by abnormality of the mind, which if successful will result in the murder charge being dismissed, thereby reducing the charge to the lesser offence of manslaughter instead.

To successfully get up on the partial defence of substantial impairment by abnormality of the mind to reduce a murder charge down to manslaughter, the accused person is required to prove each of the following elements on the balance of probabilities (i.e. more likely than not):

  1. At the time of the act causing death, the accused person’s capacity to understand event, or judge whether his/her actions were wrong or right, or control him/herself was substantially impaired by an abnormality of mind arising from an underlying condition; and
  2. The impairment was so substantial as to warrant the accused person’s liability from murder being reduced to manslaughter.

What is an ‘abnormality of the mind’ according to law? This occurs if his/her capacity to function in any one of the following ways differed from that of ordinary human beings that the reasonable person would term it abnormal:

  • Understand events
  • Judge whether the actions were right or wrong
  • Control

What is an ‘underlying condition’ according to law? This is where there is a pre-existing or underlying mental or physiological condition, that does not have to be permanent, although it isn’t enough if it’s a passing condition or one that only lasts for a short time.

In considering whether an impairment was so substantial as to warrant liability for murder to be reduced to manslaughter, the court is required to make this decision based on a value judgment applying community standards.

Any effects of self-induced intoxication from alcohol and/or drugs at the time of the actions causing death are not to be considered or regarded. They are to be completely disregarded when the court considers the above elements to this partial defence.

In the event, the partial defence of substantial impairment by abnormality of the mind is unsuccessful to a murder charge, the court will be required to return a verdict of guilty to murder.

About Jimmy Singh

Mr. Jimmy Singh is the Principal Lawyer at Criminal Defence Lawyers Australia - Leading Criminal Lawyers in Sydney, Delivering Exceptional Results in all Australian Criminal Courts.

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