A fight between cousins ended with excessive self-defence but was not murder, a judge has found.

Conrad Peter Lardner, 51, was found guilty of manslaughter on Thursday after failing to convince a judge he acted in lawful self-defence when fatally stabbing younger cousin David Vale in 2023.

The prosecution argued at the judge-alone trial in the NSW Supreme Court that Lardner should be found guilty of murder.

Both men grew up in the same house, were raised by Mr Vale's father and were part of an extended kinship network, with many other members in court to hear the verdict.

Justice Steven Campbell found there was a possibility Lardner engaged in some defensive act against the 39-year-old, but saw no reasonable justification for the violent stabbing he inflicted upon his cousin.

Lardner, who pleaded not guilty to murder and manslaughter, testified he only stabbed Mr Vale after a knife fell out of the younger man's pocket during a fistfight the two were having at a house in Kempsey, on the NSW mid-north coast.

"My wife would have been burying me," Lardner told the court when outlining what he believed would happen if he did not grab the knife and fight back.

But the Crown argued the older man armed himself with a knife after Mr Vale confronted him angrily about 15 minutes before the fist fight started.

While Justice Campbell could not definitively say how Lardner ended up with the knife, he ruled there were other options for the 51-year-old to defuse the situation.

CCTV footage from a neighbouring house showed Mr Vale walking away from his killer and towards his car before he was stabbed, Justice Campbell said.

"At no time had the deceased sought to use (the knife) ... it was the accused who followed the deceased as he was attempting to retreat," the judge said.

"The accused could have thrown (the knife) away or neutralised the presence of the knife."

The extreme force needed to inflict the fatal stabbing and the lack of defensive bruising on Mr Vale from the fight indicated any self-defence was out of proportion to the threat, the judge found.

"(Lardner) took the most extreme step available and plunged it into the deceased twice," Justice Campbell said.

"The accused is not guilty of murder but he is guilty of manslaughter."

Mr Vale confronted his killer about an unpaid debt, presumably from drugs, the court heard.

"He clearly wanted satisfaction for the debt in one form or another," the judge said.

But there was no evidence the younger cousin wanted any conflict to turn deadly, Justice Campbell found.

Lardner was remanded in custody after the verdict and will return to court for a sentence hearing on November 10.