Fremantle Dockers defender Michael Johnson has been found guilty of assaulting a man during a drunken fight at a Leederville kebab shop that began when he was filmed dropping his pants around his ankles.

Johnson was fined $3500 but given a spent conviction, meaning he will avoid a criminal record for the offence.

He stood trial in the Perth Magistrates Court last month, accused of punching 24-year-old Tim Bowen in the face in the early hours of November 13 last year.

Michael Johnson has played 231 games for the Fremantle Dockers since his debut in 2005. ( AAP: Tony McDonough )

It followed an argument between Johnson's wife and Mr Bowen, who had used a mobile phone to record Mr Johnson with his pants down around his ankles.

Johnson maintained his wife was being attacked, and he punched Mr Bowen to defend her.

In court today, Magistrate Elaine Campione said Johnson had misjudged the situation.

"You completely overreacted to the situation," Ms Campione said.

She said Mr Bowen was injured and fell to the floor.

"The blow would have caused him significant pain," she said.

But she said Johnson's behaviour was an aberration, at the lower end of the scale, and she believed a fine was the correct penalty.

Outside the court, Johnson said he was very sorry for his actions.

"I'm not happy with my behaviour at the time. But I do find the findings the right thing, and I do apologise for my behaviour, especially to the victim," he said.

"I'm not proud of my actions on the night. I've let down a lot of people, especially my teammates and my family, so I'm very sorry for my actions."

Johnson was ordered to pay $3500 for the assault, $1000 in compensation to Mr Bowen and $188 in costs.

Magistrate Campione said Johnson was generally of good character, and at low risk of reoffending.

Fremantle Dockers Chief Executive Officer Steve Rosich said the club had not yet decided whether to impose a further penalty on Johnson.

"Given the penalty imposed on Michael by the court today, any decision regarding a club-imposed sanction will be made after further discussions with our board, Michael and his manager," Rosich said.