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A young woman says she is struggling to understand how a man could punch her in the face, knocking her out, in an unprovoked attack in Perth city centre.

Police are hunting the man who attacked Ashleigh Part after he argued with her sister's boyfriend about 4am on Sunday.

Ms Part, from Kinross, had been on a night out in Northbridge and was trying to get a taxi home when she was attacked in Barrack Street.

The 20-year-old was knocked unconscious when her head hit the ground.

Unable to remember much of the incident, she has been told she approached her sister's boyfriend Josh and the stranger, and had not said a word when the man turned and hit her and then fled.

"I can't believe it could happen at all . . . but for a man to hit a woman," she said.

She has spoken out in the hope of preventing similar attacks and to ask anyone who saw her being punched to call police.

The incident comes as NSW authorities battle street violence in Sydney's Kings Cross after the death of teenager Thomas Kelly, who was fatally king hit on July 7 as he talked on his phone.

Ms Part was also knocked to the ground and lost consciousness. Fortunately, she escaped permanent injury but had a black eye and blood in her eye.

Reported assaults from July to March increased in Perth from 527 in 2010-11 to 558 in 2011-12 but have dropped in Northbridge from 424 to 326 over the same period.

McCusker Centre for Action on Alcohol and Youth director Mike Daube said reducing the availability of alcohol was a proved way to cut violence.

National Drug Research Institute Professor Tanya Chikritzhs said authorities should consider shorter trading hours and fewer licensed venues, higher alcohol prices and a higher age limit on drinking.

Police are checking security footage in a bid to trace the attacker.

He is in his late teens or early 20s, has brown-blond hair with a side fringe and wore a collared shirt.