He had viciously assaulted her before.

But 18-year-old Jazmin-Jean Ajbschitz thought she and her boyfriend, Sean Lee King, might be able to work things out when she eventually gave in to his pleas and let him into her Sydney apartment.

Within the next 26 minutes, he beat her to death in a ferocious, drug-fuelled attack - hitting her with several objects before stomping on her chest until her heart stopped beating.

It took a jury less than two hours on Monday to find him guilty of her murder.

King, 27, had admitted to killing Ms Ajbschitz on July 10, 2011, but he denied murdering her, on the grounds that he was high on the drug ice at the time and did not intend to kill her or cause her serious harm.

During the three-week trial, the jury was told King had bashed Ms Ajbschitz, known as Jazy, several times during their turbulent 16-month relationship, including a beating in front of his family.

She suffered bruising to her arm, pain in her chest, a black eye and head injuries during those assaults, the court heard.

King admitted he had previously punched Ms Ajbschitz in the ribs, but he told the jury he didn't think that counted as "serious violence".

On the night he murdered her, he phoned her several times and threatened to kill her and everyone with her.

A scared Ms Ajbschitz called her mother, who had moved to Perth just five days earlier, and told her she was afraid King would break into the apartment.

But eventually King used his "powers of persuasion" on Ms Ajbschitz, who was seven years younger than him, and she told her anxious friends, "Me and Sean will sort things out."

CCTV footage showed some of the last moments of Ms Ajbschitz's life in which she hesitated at her front door, arms folded, as King paced around and begged to be let in.

Eventually, she opened the door and tried to hug him.

He shrugged her off.

And followed her inside.

Over the next 26 minutes, he beat, punched and kicked her.

There was evidence he dragged her bleeding down the hallway and stuck her head down the toilet.

He finally stomped and possibly jumped on her as she lay on the kitchen floor.

King left the building with his head hooded and his sleeves pulled over his hands, before telling his friend to dial triple-zero from a phone box.

Paramedics couldn't find the right unit, however, and Ms Ajbschitz's brother found her partially clad body three days later.

At one point, King told a forensic psychiatrist he remembered hitting Ms Ajbschitz once that night, but when he took the stand he told the jury he had no memory of what happened inside the apartment because he had smoked so much ice.

However, he had only one answer to a series of questions from crown prosecutor Kara Shead.

Was he responsible for each and every injury inflicted on Ms Ajbschitz?

Did he attack her with great ferocity?

Did he threaten to kill her and then do just that?

"Yes," he said.

Ms Ajbschitz's mother, Anne Larter, described how she tried to persuade her "smart, bright, intelligent" daughter that she could do so much better than King.

"She used to listen and she used to agree," Ms Larter said.

Ms Ajbschitz did make one final attempt to break up with King just hours before he murdered her.

"You're a cheating, girl-bashing dog," she told him.

It was in stark contrast to a text she sent him one week earlier.

"I miss you," she told him.

"You make me so happy lover."