Robert Karl Huber, 69, had been in a volatile relationship peppered with domestic violence against Lindy "Kimmy" Lucena, whom he killed on January 3, 2023.
Huber was convicted of manslaughter in the NSW Supreme Court on Monday.
But he was acquitted of the more serious charge of murder.
The couple had been staying in the courtyard behind a Salvation Army store at Ballina in the NSW Northern Rivers region after becoming homeless following floods in 2022.
Justice Stephen Rothman found Huber committed an "unlawful and dangerous act" when the 69-year-old bashed his partner and should have appreciated the risk of serious injury.
But medical expert evidence tendered at trial acknowledged Ms Lucena did not die from the wounds she suffered during the assault.
"Death was not the obvious and inevitable consequence of the assault by the accused of the deceased," the judge said.
Huber had not intended to commit injuries so serious that they elevated the killing from manslaughter to murder, Justice Rothman found.
While the motive was unknown, the judge pointed to his anxiety and depression about being homeless, his "irrational blame" of Ms Lucena for their situation and his overall feelings of disempowerment.
Found with blocked or restricted arteries, Ms Lucena could have died of an abnormal heartbeat as a result of the brutal assault, the judge said.
Small amounts of methadone found in her system could have contributed to her death.
Justice Rothman accepted Huber had assaulted Ms Lucena about 7pm on January 3, after hearing evidence from passerby Christopher Daley at trial in June and the triple zero call he made at the time.
Police took almost an hour to respond after having difficulties locating the scene and found nothing.
At 7.20pm, Lesley Hudson-Moon was walking her dogs when she saw Ms Lucena lying in the courtyard through slats in the fence.
She might have already been dead, Justice Rothman said.
Huber had returned from the pub when he attacked her, moved her body and attempted CPR, before possibly falling asleep.
CCTV footage shown to the court showed him visit Ballina police station after midnight, leading officers to the courtyard where Ms Lucena's body lay.
Huber earlier pleaded guilty to a separate assault on December 26, 2022 when he gave Ms Lucena a black eye.
He remains behind bars and will face a sentence hearing on September 12.
1800 RESPECT (1800 737 732)
Lifeline 13 11 14
Men's Referral Service 1300 766 491
7°C