Tabalbag's hands were small enough for him to reach inside and undo the chain. Had they been larger, Amin Sthapit might still be alive.

Tabalbag, 30, said he was "a demon possessed" as he chased Mr Sthapit around the tiny Darling Point unit, stabbing him to death with a chef's knife.

"It wasn't me," he had told the NSW Supreme Court in Sydney.

Justice Michael Adams agreed with the convicted murderer on Monday as he sentenced Tabalbag to a minimum of 12 years' jail and a maximum of 17 years.

"The crime was not premeditated but impulsive, committed when Mr Tabalbag's powers of reasoning, judgment and self-control were temporarily but significantly disordered," Justice Adams said.

"It was completely out of character and should be regarded as a surprising, though tragic, aberration."

Tabalbag, who grew up in the Philippines, had been a good student and an attentive partner, the court heard during his trial.

He and his then girlfriend Geecy Rebucas, 30, had moved together to Australia in 2007.

Five years later, Tabalbag had become suspicious Ms Rebucas was cheating on him. He used mobile phone and computer programs to try to read her messages and track her phone's location.

One tracking program pointed to their flat on a Monday morning when Ms Rebucas had said she would be at her cooking school.

Instead, she was at home with her lover, Mr Sthapit, also 30, who was her boss at the Double Bay hotel where she worked.

When Tabalbag entered the room, he grabbed a box containing a chef's knife that Mr Sthapit had given Ms Rebucas as a present.

Tabalbag stabbed his victim 13 times, including seven times in the back, while screaming: "Why?"

As Justice Adams detailed this "frenzied attack" in his judgment, Tabalbag sat in the dock with his eyes closed, crying silently.

Mr Sthapit's fiancee, Pradeepa Yonzon, stared straight ahead.

In sentencing submissions last week, she said she had forgiven her partner for his infidelity and had lost her best friend.

"In this life, nothing is promised to us, nothing belongs to us," Ms Yonzon said.

"But why this? Why us?"

Tabalbag pleaded not guilty to murder but guilty to manslaughter.

His defence said the attack was provoked and an ordinary person could have acted the same way in the circumstances.

But after one juror was discharged in June, claiming he could not reach a verdict due to his Buddhist beliefs, Tabalbag was found guilty of murder by a majority of 10 to one.

Taking into account time already spent in custody, he can first apply for parole on November 10, 2025.