Sewell, 29, appeared in Melbourne Magistrates Court on Tuesday where he was found guilty of recklessly causing injury and affray over a March 2021 altercation outside Nine's Melbourne headquarters.
After two days of contested hearings last week, magistrate Stephen Ballek asked Sewell to stand in court as he delivered his verdict and dismissed Sewell's arguments of self-defence.
"I find you, Thomas Sewell, guilty," he said.
Mr Ballek said Sewell was "itching for a fight" when he visited Nine Network's building on March 1 with a cameraman and demanded to speak to someone from A Current Affair.
He claimed ACA's segment on him that evening showed his group to be a terrorist organisation and said the program had not contacted him for comment.
After ACA staff declined to meet with Sewell, his cameraman Jacob Hersant began filming Sewell inside the foyer.
A security guard walked over, placed his hand over the camera lens and told the pair to stop filming, directing them to an area out the front of the building.
The pair eventually agreed to go outside, but the guard followed them after they started filming near the door. They finally listened to the man's directions and moved further away.
The guard then began making a dancing-type motion to the camera.
Mr Hersant taunted and mocked the guard, saying: "Dance monkey, dance".
The guard told him "watch yourself bro, I'm not a dance monkey" and touched Mr Hersant on his shoulder, in an effort to push him backwards, before Sewell leapt in and began attacking the guard.
The magistrate said the guard had no time to react when Sewell began punching him in the head.
The "force and repetition" of the punches caused the guard to fall backwards and hit his head on the concrete, Mr Ballek found.
He rejected Sewell and Mr Hersant's evidence, which included that the guard had grabbed Mr Hersant by the throat and Sewell had beaten the guard in an effort to defend his friend and himself.
"It seems to me that the very second you perceive any physical contact from [the guard] on Mr Hersant, you leapt in with sustained and unjustified violence," Mr Ballek said.
"The video evidence graphically shows the disturbing nature of a strong man brutally punching an unsuspecting victim in the face, such that he falls backwards and strikes his head on the pavement."
An alternative assault charge was thrown out after Mr Ballek found prosecutors had successfully proven recklessly causing injury.
Sewell, who remains on bail, is facing up two years in jail. He will return to court on January 12 for a pre-sentence hearing.
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