A central Wellington alleyway where a 21-year-old woman was assaulted early on Saturday has been a notorious problem spot, police say.

Today they will seek CCTV footage from businesses near Opera House Lane, which leads from Manners St to Wakefield St, in an attempt to identify the woman's attacker.

She was walking through the lane about 2.30am when she was grabbed by a man.

He pulled her into the corner, Detective Andy Compton, of Wellington CIB, said.

"The young woman was pinned against the wall by the offender and had her dress ripped before she was able to fight him off, causing him to run away."

The victim told the man she was ringing the police while she fought him off, but rang her brother who was just across the road.

The brother ran down the alleyway to find her, calling police as he went.

The dark alleyway had been a problem area in the past, Mr Compton said.

"Historically, it is an alleyway where there has been assaults."

However, there had not been any recently and it was fairly well used.

Mr Compton described the woman's attacker as European of heavy build.

He was about 180cm tall and wearing a dark or black hooded sweatshirt.

He hoped someone had seen the incident, or the moments leading up to it, and was urging people with information to ring police on 043812000 or the free Crimestoppers hotline on 0800555111.