Anger over a woman led to a Northland man punching his stepfather, who later died in hospital, a jury has been told.

Patrick Tepana, 44, denies a charge of manslaughter over the death of Christopher Edwards, 61, at Te Kopuru in Kaipara, early last year.

Addressing the jury in the High Court at Whangarei yesterday, Crown prosecutor Anna Patterson said the Tepana family's lifestyle involved heavy alcohol intake almost on a daily basis.

She said that while Mr Edwards was a quiet and unassuming person, Tepana was a jealous and sometimes abusive individual, especially when his jealousy was sparked over his partner.

His jealousy, she said, had been festering for over a week and boiled over sometime between late on January 10, 2012 and lunch on January 11. Tepana punched Mr Edwards three to five times with considerable force while both were drinking.

Ms Patterson said Tepana assaulted his stepfather at their West Coast Rd home, about 13km southeast of Dargaville, because he thought Mr Edwards had stirred up his girlfriend.

Mr Edwards did not offer any resistance and later retired to his bedroom while the rest of the group continued drinking, she said.

When a family member went to wake Mr Edwards for a meal, his bed was bloodstained and he was clearly unwell.

St John paramedics noticed he was unresponsive, appeared to be snoring and was blue in the face.

Mr Edwards was taken to Dargaville Hospital then transferred to Whangarei, where a CT scan showed a large subdural haematoma - or collection of blood - on the surface of the brain, beneath the skull.

He was flown to the Auckland City Hospital. However, he died on January 13, 2012 after his life support was switched off.

Ms Patterson said Tepana repeatedly denied to police that he had assaulted Mr Edwards or said that he couldn't remember.

At one point he said the assault was like "three smacks".

Tepana had abrasions on his knuckles which were consistent with him punching his stepfather, she said.

He asked a police officer why he was being interviewed.

"I only punched him," the jury heard.

Defence lawyer Chris Muston said the case was very unclear and there would be issues around the reliability of witnesses, some of whom were thoroughly inebriated, and how and when Mr Edwards suffered the injuries.

The case continues today.

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