A GOOD Samaritan who rushed to the aid of a woman "having her lights punched out" was gobsmacked to get a $553 fine from police in the post.

Aaryn Hayes, 25, was with friends in Brunswick St, Fitzroy, on Sunday morning when he saw a man grab a woman by the throat and throw her to the ground.

Mr Hayes grabbed the man towering over her and pinned him against a wall.

Seconds later, police arrived and doused both men with capsicum spray.

"The guy was punching the hell out of this lady," Mr Hayes said.

"I didn't punch him or hurt him. I simply grabbed him and moved him away from her to stop the violence.

"She was having her lights punched out - he was getting really stuck in - and people were just walking past. I had to help," he said.

Mr Hayes said while one police officer dealt with "the yelling and screaming offender", he explained to the other what had happened, and they helped him clear the spray from his eyes.

"Then the other officer came over and said I was disturbing the peace and acting in a riotous way.

"I was bewildered. I was just trying to stop a woman being beaten to a pulp and police were saying I was in the wrong for helping.

"I was with seven friends and my girlfriend, who were all witnesses. But when they spoke to the officer he threatened to arrest me and take me back to the station.

"Meanwhile the guy who attacked the woman was let go because the woman didn't want to press charges."

Mr Hayes said his shock was compounded on Tuesday when he received in the post a $600 penalty notice for "riotous behaviour".

"I feel like a criminal for trying to help police," he said.

"I keep thinking, 'What if it was my girlfriend or mum who was being attacked, and no one helped?'.

"It's not in my personality to walk by, but the fine has made me think twice."

Mr Hayes said he was seeking legal advice and would contest the penalty in court.

A Victoria Police spokesman, Belle Nolan, said: "It's believed a man in his 20s intervened in a verbal dispute between a man and a woman. It's alleged the man assaulted the victim, aged in his 50s, and refused to comply with police instructions to stop.

"He was sprayed with capsicum spray and received an infringement notice for riotous behaviour."

jon.kaila@news.com.au