A Dunedin man allegedly beat his flatmate to death over comments about his terminally-ill sister during a heavy drinking session, a court has heard.

Jason Karl Blackler (48) is on trial before the High Court at Dunedin where he pleaded not guilty to the manslaughter of 66-year-old Alan James Fahey, as well as an alternative charge of wounding with intent to injure.

Fahey was found on October 26, 2016 - the day after his death - face down on the floor of his Brockville Rd lounge "soaked" in blood. A mutual friend of the pair, Stephen Ferguson, had allegedly been directed to the address by Blackler, who said he "might have killed God".

"God" was the nickname of the victim, from the acronym: "grumpy old decorator".

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Ferguson described Fahey as "ice cold" to the touch.

Crown prosecutor Richard Smith told the jury this morning that the defendant and victim were living together at the time and Fahey had been to the liquor store on the morning of October 25 where he bought a bottle of Jagermeister.

The pair spent much of the day consuming it, along with beer, and Ferguson, who spent a couple of hours with the men in the afternoon said the atmosphere deteriorated because the victim had a migraine.

"Alan didn't appear to be his usual jovial self," he said.

At 10.30pm, Blackler made two phone calls to his partner, Smith said.

Just after midnight, he got a taxi to her Corstorphine home.

The next morning, after summoning Ferguson to the house, the defendant asked him to go to the Brockville Rd address to check on Fahey.

There he found the corpse and went back to Blackler to pass on the news.

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Ferguson pushed him to give himself up to police but instead the defendant instead asked for a lift into town.

The pair went to Community Probation where Blackler asked to speak to a specific officer.

When he was told that person was not available they left, but Ferguson had told another staff member about the grave circumstances of their visit.

Blackler then went to Credit Union, Smith said, to withdraw cash.

There he allegedly told a teller he had beaten a man to death.

Police picked Blackler up shortly afterwards on Moray Pl where he allegedly told an officer it was "a one-punch thing".

In an interview at the station, the defendant explained Fahey had made disparaging comments about his sister that night but claimed he could not remember an assault.

Smith said forensic examination of the scene showed bloody footprints leading from the victim's body to the bathroom, where there was more blood present in the basin and shower.

Blackler's shoes were seized and traces of blood almost certain to be Fahey's was found, the court heard.

Smith told the jury the victim suffered a large cut from his lip to his nose, bruising around the face, nasal fractures and a fracture of a bone in the throat.

A post mortem discovered Fahey suffered lung disease from smoking and severe coronary-artery disease.

His blood-alcohol reading was more than seven times the legal driving limit.

Smith said, regardless of the pre-existing medical complaints, an assault on the victim was "a substantial and operative cause of death".

"It was, for want of a better phrase, the trigger," he said.

Defence counsel Anne Stevens said there was no conclusive proof that her client attacked the victim or that an assault had even taken place.

"What you do know is we had an older, angry, grossly drunk man in a house you will see was so messy it was hazardous," she said.

"In that state, in that house, he could have fallen over and injured himself at any point."

Stevens said her client's comments to police came when he was highly emotional and intoxicated.

The trial before Justice Rachel Dunningham and a jury of eight men and four women is scheduled to last two weeks.