Jason-Manuel Zerafa, 26, was arrested on August 29 and charged with one count of common assault after a fight at Sydney's International Convention Centre the night before.
He appeared with his lawyer Upol Amin at Downing Centre Local Court for the first time on Monday where he immediately pleaded guilty.
Jason-Manuel Zerafa threw a punch at trainer Tommy Mercuri after the two corners exchanged words. (HANDOUT/NO LIMIT BOXING)
"A boxing ring is usually a place for lawful violence, but this is a situation where we've got unlawful violence in the boxing ring," the solicitor told Magistrate Chris Longley.
His brother Michael Zerafa scored a technical knockout against Tommy Browne but the win was overshadowed when Jason-Manuel Zerafa threw a punch at Browne's 60-year-old trainer Tommy Mercuri after the two corners exchanged words.
Browne suffered a bicep injury in the first round of the bout and did not come out of his corner for the second frame - a move that appeared to enrage his opponent.
Mr Longley did not convict Jason-Manuel Zerafa on Monday, but imposed a six-month conditional release order for the assault.
He said that although blood was thicker than water, that still did not permit the younger sibling to act as he did with an "element of stupidity".
The court was told Jason-Manuel Zerafa acted after seeing the trainer "square up" to his brother. (HANDOUT/NO LIMIT BOXING)
He also noted an apology had been sent to Mr Mercuri through a letter filed with the court.
Earlier on Monday, Mr Amin said his client had jumped over the ropes after seeing Mr Mercuri "square up" against Michael Zerafa and assumed he would throw a punch.
"He saw his brother Mr Michael Zerafa's hands down and an ungloved experienced fighter squaring up," the solicitor said.
Words were exchanged between the 26-year-old and the trainer in the spur-of-the-moment incident before it turned violent.
Mr Amin said his client, who worked as a security guard and studied nursing, was a man of good character who did not have any criminal convictions anywhere.
He had also suffered by spending six hours in custody after the arrest, missing his flight back to Victoria where he lives, and being subjected to negative social media posts after news outlets reported on the assault, Mr Amin told the court.
Upol Amin said his client was of good character, working as a security guard and studying nursing. (Bianca De Marchi/AAP PHOTOS)
Mr Mercuri suffered minor injuries which did not require treatment, police said after the assault.
The police prosecutor unsuccessfully argued for a conviction, saying that because the fight had been beamed around the world and seen by countless people, it brought the sport of boxing into disrepute.
"There's a sense of irony there I have to say," Mr Longley replied.
The sergeant responded, saying there was a community expectation that people in a boxer's corner, who were seen by the public to be there as trainers, should not enter the ring to start fights themselves.
Promoter No Limit Boxing confirmed it issued Jason-Manuel Zerafa a life ban for the "unacceptable" incident.
The fight was the co-main event on a card headlined by Nikita Tszyu's ninth-round TKO win against Koen Mazoudier.
Michael Zerafa fought for the WBA middleweight title in March but was stopped by Cuban Erislandy Lara.
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