Rita Camilleri's head was found on the footpath, while her decapitated body was on the kitchen floor as were her eyeballs, tongue and tip of her nose, prosecutor Tony McCarthy said on Tuesday.

Family friend Jade Arena later testified to telling Ms Camilleri two weeks before her death "that girl is going to murder someone", to be told "Jessica would not hurt a fly".

Jessica Camilleri, 27, has pleaded not guilty in the NSW Supreme Court to murdering her mother at their St Clair home in July 2019.

Both the prosecutor and her barrister Nathan Steel agree Camilleri stabbed her 57-year-old mother, her sole carer.

But the question for the jury is whether the charge could be reduced to manslaughter due to the partial defence of being unable to control her actions due to a substantial impairment of the mind.

In his opening address, Mr McCarthy said the mother was said to have been "overly protective and defensive" of her daughter's behaviour, often placating her to avoid conflict.

She had suffered social, behavioural and learning difficulties including ADHD since she was a child, and was later diagnosed with conditions including anxiety, rage disorder, and autism spectrum disorder.

She had a history of assaulting people, some involving random victims and others relatives, as well as of harassing and repeatedly threatening strangers over the phone, the prosecutor said.

Kristi Torrisi told the jury her younger sister constantly watched horror movies with her favourites being The Texas Chainsaw massacre and Jeepers Creepers.

"She liked horror movies where there was killing and dismemberment," she said.

Her sister had a bad temper and was very demanding of their mother.

Ms Arena said Ms Camilleri had been very submissive towards her younger daughter as she didn't like dealing with her tantrums.

Mr McCartney said that on night of the stabbing, Camilleri rang triple-zero saying her mother had had enough of her and got a knife out and tried to stab her, before Camilleri grabbed it from her.

Mr McCarthy said the jury would hear the audio of the call in which she said: "I just kept stabbing and stabbing and stabbing her, I took off her head".

"She also said she took her mother's head with her to show her neighbour," he said.

Camilleri reported there being all sorts of knives, seven of them, some which had broken.

"She repeatedly asked if someone could be brought back to life if their head was cut off," the prosecutor said.

In a recorded police interview, Camilleri said her mother wanted to send her to the mental health unit at Nepean Hospital and had told her "I've had a gutful", grabbed her by the hair and threatened her with a knife.

She estimated she had stabbed her mother 85 times, saying she wanted to give her "a taste of her own medicine".

Mr McCarthy said the Crown submitted the mother, not her daughter, had been acting in self-defence.

Ms Torrisi told the jury of numerous random threatening "prank calls" made by her sister, including many to a Melbourne man which resulted in their mother reporting them to police.

Her mother tried to get help for Camilleri but used to say the mental health system was letting her down.

Ms Torrisi became worried about her mother becoming exhausted, and suggested her sister be moved to a group home for more support.

The trial continues before Justice Helen Wilson.

Australian Associated Press