Jessica Silva made a recording of her partner James Polkinghorne abusing her and sent it to her best friend so it could be handed over to police "in case anything happens".
Ms Silva is charged with murdering Mr Polkinghorne by stabbing him five times outside the Silva family home on May 13, 2012.
She has pleaded not guilty on the grounds of self-defence.
On Friday, Forever Tupou told a NSW Supreme Court jury that she and Ms Silva had been friends since they met in year 7 at school.
Ms Tupou said Ms Silva initially hid the verbal and physical abuse she suffered at the hands of Mr Polkinghorne but eventually confided in her.
Ms Silva showed Ms Tupou expletive-filled text messages Mr Polkinghorne regularly sent her.
Ms Tupou also witnessed him berating her and "muttering" strange comments in front of other people at the unit the couple shared in Dulwich Hill.
"It was really scary," Ms Tupou said.
At one point, Ms Silva used her mobile phone to record Mr Polkinghorne threatening to "cave in her skull" and throwing objects around the unit. She sent it to Ms Tupou's phone.
"I wanted to take the recording so I could have something to show the police in case something happened to her," Ms Tupou said.
Ms Silva's neighbour Meleane Fisiitalia said the 24-year-old told her she feared for her life and that of her family because Mr Polkinghorne had threatened to hurt her and them if she ever reported his behaviour to police.
Ms Fisiitalia said she saw bruising to Ms Silva's neck, ear and arm allegedly sustained when Mr Polkinghorne followed her to Brisbane and bashed her a few weeks before his death.
But Ms Silva wore her hair down to prevent her parents from seeing her injuries, Ms Fisiitalia said.
Wiping away tears, Ms Fisiitalia said Mr Polkinghorne was taking illicit drugs and Ms Silva said to her, "I thought he was the one but now he has got out of control."
Methylamphetamine, also known as ice or crystal meth, was found in Mr Polkinghorne's blood during a post-mortem examination, the court heard previously.
After Ms Fisiitalia's evidence, Crown prosecutor Eric Balodis closed the prosecution case.
Gregory Scragg will open the defence case on Monday.
