Melb assault not racially motivated: cops

There is no suggestion that an assault on a Muslim woman in Melbourne's CBD was racially motivated, police say.

The 21-year-old woman was walking down a CBD street on Thursday afternoon, when a shirtless man punched her in the head, causing her to fall and hit her head on the pavement.

The man said nothing to her before the attack and police say the victim, who was treated at the scene for minor injuries, was surprised to see media reports suggesting it was racially motivated.

Islamophobia Register Australia said a 16-year-old Muslim boy stepped in to stop the attack but police refute this claim.

They say an unknown boy did speak to officers at the scene but he was creating a "stir" and swearing while making certain allegations.

Islamophobia Register's president Mariam Veiszadeh says the boy told her that he chased the man after the incident and he produced a knife.

She says police had not bothered to follow this up.

"There are actual witnesses to this who have come forward and (police) have effectively discounted his account," Ms Veiszadeh told AAP on Friday.

"I understand that we have to exercise restraint and not jump to conclusions about the motivations of the offender and we have not sought to do that."

Ms Veiszadeh said racial motivation could not be ruled out as the woman was wearing a hijab and visibly Muslim, and given the current climate it is not "far fetched" for people to be asking questions about the offenders possible motivation.

The bearded attacker, who has a tattoo on his upper right arm, was captured on CCTV footage but there is no CCTV available of the assault.

Police are appealing for public assistance to identify the man.