A man who punched another man outside a Northbridge nightclub, leaving him with permanent brain injuries and confined to a wheelchair, has been found not guilty of causing him grievous bodily harm.

Connor Pestell, 24, said he had acted in self-defence when he punched Mitchell Cleary outside the venue in William Street in the early hours of March 2 last year.

Mr Cleary was left with serious injuries when he hit his head on the ground after being punched twice by Pestell.

The jury at Perth District Court found Pestell guilty of assaulting Mr Cleary's friend, Deniz Varol, who suffered a cut lip.

Pestell, a trained boxer, was fined $1,500 for the assault.

The court heard Pestell punched Mr Varol after he abused one of Pestell's friends.

Pestell told the court Mr Varol was being "abusive and aggressive" towards his friend, who was heavily intoxicated.

Jurors heard he told Mr Varol "if you come near me or him, I'll chip you", before punching him in the face.

"I thought he was going to attack me or (my friend)," he told the court.

Pestell said he then took two or three steps backwards before he was confronted by a group of people, including Mr Cleary, whom he maintained had tried to punch him.

"I recall his arm going... towards my head," Pestell said.

Pestell then punched Mr Cleary twice.

Mitchell Cleary was left brain damaged and confined to a wheelchair after the fight in 2013. ( Facebook )

"He's already swung at me, once. I had him and a couple of people running at me. I didn't know what to do," he said.

"I wouldn't say hard, but I did punch Mr Cleary in self-defence."

The altercations were captured on security cameras.

Under cross-examination, Pestell conceded he knew it was dangerous to punch someone who had been drinking alcohol.

"Of course it's dangerous, but I was acting in self-defence. What if he punched me? I could have been the one who fell backwards," he said.

Pestell denied that in both altercations he had lashed out in anger.

Members of Mr Cleary's family, who attended every day of the trial, made no comment as they left the court.