They failed to successfully shoot any of the ducks, and as they turned to leave Mr King's gun went off.
He had forgotten to put the safety back on, his lawyer Paul Borich said, and the shot hit his friend in the elbow.
"It discharged because his finger was around the trigger area. He's lucky that all that happened was an injury to the elbow by that sort of accident, rather than something a bit more horrendous," Mr Borich said.
He had been in regular contact with his friend, who has undergone surgery on his elbow.
Mr King grew up with the victim in Hawke's Bay, and the group of old school friends met up every year for the duck shooting trip.
Injuring his longtime friend was "something that bears heavily on Mr King", Mr Borich said.
He had spoken to him three or four days ago and had received an update on his recovery, which was described as "a lot more positive than portrayed" in court documents.
The married father-of-three stood silently in the dock, dressed in a black suit and blue tie.
It looked likely his firearms licence would be revoked following the incident, Mr Borich said, and the hunter had no issues with that.
"I think the experience perhaps put him off shooting, certainly for the short-term."
The police prosecutor described the case as unusual, saying the court had not dealt with one for some years.
Mr King's bail was continued until July, when he will appear for sentencing.
