The officer's M4 Bushmaster rifle unexpectedly discharged as he leant forward to help lift the man from the ground when the AOS operation had come to an end.
The man was critically injured and taken to Wellington hospital after the bullet entered his shoulder though a "thick woolen jersey".
The squad was called to the Karamu Rd home the man shared with his mother after she had called with concerns about her son's behaviour and mentioned there was a gun in the house.
The man came out of his house responding to the loud speaker which was used. He was yelling aggressively and wearing boxing gloves but complied with police and was restrained and handcuffed with plastic cable ties.
He had been tasked with reading the man his rights which he did while kneeling lightly on his back with his left knee.
The constable had slung his rifle to his right side, after checking the safety catch was on, so he could kneel.
On standing, he returned the rifle back to his front, believing the safety catch was still on and did not recheck it.
Usually a click could be heard when the safety catch was moved. During his evidence, the officer said he had not heard anything.
"I've gone forward a little basically, tilted a little bit, that's when I heard the bang," he said.
He said he did not realise it was his rifle that had discharged at first, usually smoke or a flash could be seen, "I don't remember seeing anything".
Throughout the trial the court heard from fellow squad members, police and forensic scientists who described events leading up to the unintentional discharge.
A national forensic scientist took a rifle from a fully suited AOS mannequin which had been rolled into the court room for evidence and demonstrated how the weight of the rifle was such that if the switch rubbed on something it could be flicked from the safety setting to fire setting.
