'How could you be so merciless?': Ex-girlfriend knifed 58 times after balcony fall

When Shuyu Zhou fell from the balcony of her former girlfriend's apartment in Zetland in June last year, she was badly injured but very much alive.

As the 23-year-old lay helpless and writhing in pain, and while a passer-by called for help, Zixi "Jessy" Wang descended from the building, took a 13 centimetre knife from her pocket and inflicted 58 wounds on her former lover, known to friends as Lianne.

Graphic CCTV footage of Ms Zhou's final moments – totalling 11 distressing minutes – was played to the NSW Supreme Court on Monday during a sentencing hearing for Wang, who pleaded guilty to the murder in May.

In the footage, Ms Zhou is seen falling onto a corrugated metal fence and then onto the ground. Her legs and arms are flailing but she is unable to get up, having sustained a dislocated hip and fractured lower spine. A pathologist later concluded the injuries sustained in the fall "would not have been immediately fatal", according to court documents.

Minutes pass and Wang is seen leaving her apartment via a lift, walking down to where her former lover lies injured. She fiddles with the injured woman's clothing, then returns upstairs, checking her hair in the mirror of the lift, and changes her own pants. She does not call an ambulance or attempt to help Ms Zhou.

In the meantime, a passer-by finds Ms Zhou and calls triple zero. While he is on the phone, Wang returns and can be seen relentlessly striking the helpless woman in the neck, arms and upper torso, for more than two minutes. Ms Zhou's screams can be heard in the background of the triple zero call as the passer-by retreats in fear for his own safety.

Crown prosecutor Christopher Taylor told the court that, according to agreed facts, before she fell, Ms Zhou had been in a physical fight with Wang inside the apartment in June last year and Wang "did not wish to let the deceased leave the apartment".

Her blood was later found in the lounge room, kitchen and balcony door of the apartment – while Wang sustained a comparatively "minor" bite to the cheek, Mr Taylor told the court.

Ms Zhou's mother Li Wang, who left the courtroom while the CCTV footage was played, returned sobbing to describe the impact of the murder on her family, who moved to Sydney from China in 2009.

Speaking through a Mandarin interpreter, she told the court her daughter had moved out of the family home only a month before she was brutally killed.

She described a young woman who had studied philosophy at Macquarie University, loved to play the piano and sing, and was "kind, healthy, beautiful and very happy to help people".

"I am now living my life as good as dead ... I’m hoping to see the day when the murderer gets the justice she deserves," Ms Zhou's mother said.

"I just want to ask the murderer, 'How could you be so merciless?'"

Wang's barrister Anthony Bellanto, QC, submitted that his client's actions were out of character due to deteriorating mental health as a result of the relationship ending and Ms Zhou beginning a new relationship with a mutual friend. But he said a suggestion by the Crown that Wang was the aggressor during the fight in the apartment was "speculative".

He said mutual friends observed that after the break-up Wang "became depressed, lonely, and her sleeping pattern was affected" and she told acquaintances "she wasn't coping".

Mr Bellanto said Wang had "taken steps to address what was unfolding" and, according to her psychiatrist, was manifesting severe symptoms of a major depression.

Justice Robert Beech-Jones will hand down the sentence next month.