David Tran is facing trial for the murder of his former lover, Daiane Pelegrini, 33, in 2020.
Her new boyfriend, Steven Sameer Salman Qaqos, told the Supreme Court Ms Pelegrini complained in the months leading up to her death of being harassed by Tran.
On the first occasion she talked about Tran, she told her new boyfriend "he'd been chasing her and he was not leaving her alone".
Tran allegedly became desperate to reignite the relationship after it had disintegrated, and flew into a jealous rage after breaking into her west Sydney apartment and discovering her with Mr Qaqos.
He is accused of fatally stabbing Ms Pelegrini on August 3, assaulting Mr Qaqos with a knife and unlawfully entering her home intending to stalk or intimidate.
Several times he saw Ms Pelegrini hang up on Tran's phone calls before putting her phone on silent, Mr Qaqos told the court.
In the months before her death, Tran began arriving at Ms Pelegrini's home and loitering on the stairs.
About two months before her death, Mr Qaqos spotted Tran's black Subaru outside the home, and saw him walk up the stairs to her home.
Four days before the alleged murder, Tran was at the home again, as Mr Qaqos and Ms Pelegrini returned home after a shopping trip.
Ms Pelegrini called police, telling officers: "David is now sitting on the stairs in front of my unit, and I want to get into my house."
Her ex-partner, Marcelo Antunes De Ataide, told the Supreme Court through a translator Ms Pelegrini always left the back door of her Sydney home open.
"Because she believed that Australia was safe," he said on Tuesday.
Ms Pelegrini originally met Mr De Ataide in Brazil in 2007, and the pair later had a child and moved to Australia in 2016 as she aspired to become an aged care nurse.
Their relationship began to fray in after February 2020, although the pair continued to spend time together and remained intimate in the days up to her death.
About this time, the victim met Mr Qaqos on an online dating app, and they began exchanging messages, photos and videos.
The pair first met in person in March and the relationship quickly developed from there.
At the same time, texts exchanged between the victim and Tran show them arguing over whether or not she was his "girlfriend", and Tran making unannounced visits to her home, crown prosecutor Rossi Kotsis previously told the court.
On one occasion he arrived unannounced at a holiday the mother took at Bundeena, south of Sydney.
The court was also played a body worn camera video of a walk through of the crime scene, which showed the blood-soaked Oatlands home after the alleged attack.
Puddles of blood were seen on the living-room carpet where paramedics had attempted to save Ms Pelegrini before she died at the scene.
Mr Tran's lawyer Nicole Carroll said the defence would not dispute many of the facts put forward by the prosecution, but would argue her client was mentally ill when he killed Ms Pelegrini.
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