Jessica Camilleri was found guilty of manslaughter and sentenced to a minimum of 16 years and two months in jail in March 2021 after stabbing her mother, cutting off her head and removing her eyes in July 2019.
She was also given a full jail term of 21 years and seven months, expiring in 2041.
Justice Helen Wilson found the horror movie enthusiast suffered from a substantial impairment of the mind when she lost control in a "violent rage" on the day she stabbed her mother to death and carried the decapitated head out onto the street.
Camilleri was diagnosed with a myriad of mental health conditions over her lifetime, including poor anger control, impulsive behaviour, anxiety, panic disorder and intermittent explosive disorder.
She was prescribed mood stabilising and anti-psychotic drugs but in April 2019, months before the attack on her mother, was not taking her medication and had opted to see a naturopath and undertake hypnotherapy instead.
On Wednesday, Camilleri managed to reduce her sentence by five years in the NSW Court of Criminal Appeal, despite judges acknowledging the attack was "gruesome and brutal".
Two out of three judges found Justice Wilson erred when assessing the gravity of the killing by giving Camilleri's mental state and loss of control less weight than she should have when handing down the sentence.
"(In) dealing with an offence of manslaughter following a plea of guilty or verdict based on a finding or acceptance of substantial impairment, the sentencing judge should not diminish the role played by an offender's cognitive or neurological impairment or mental illness," said Justice Peter Hamill.
The judge said Camilleri's moral culpability was "significantly diminished" due to her mental condition.
"I find (Camilleri) had only a simple understanding of moral wrongfulness, due to her intellectual disability and autism spectrum disorder, and had lost her self-control at the time of the homicide," he wrote.
The appeals court also reduced the sentence after finding Justice Wilson did not offer a discount of more than 10 per cent despite Camilleri's offers to make an early guilty plea to manslaughter.
She has now been given a non-parole period of 12 years, expiring on July 19, 2031.
Her full prison term will expire on January 19, 2036.
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