Police have announced a $500,000 reward for information which leads to the arrest of a man who raped a teen in a Melbourne CBD laneway.

Amy*, who was 18 at the time of the assault, was walking along Flinders Street towards Swanston Street to catch a tram after being out celebrating a friend’s birthday.

As she walked, a man appeared from a doorway leading from United backpackers on Flinders Street and began trying to engage her in small talk before placing an arm around her shoulder.

Victoria Police are offering $500,000 for help locating a man who can assist in the investigation into a 2018 sexual assault in a Melbourne CBD alleyway. (Victoria Police)

Amy tried to get away, but as the pair passed the Metro Tunnel works on Swanston Street the man dragged her into an alleyway behind high metal gates. He locked the gates and raped her.

She was able to scream, and managed to break free, running from the man and out past the gates onto Flinders Street.

Amy, now 19, has spoken for the first time since the traumatic attack in a bid to find the attacker and said she recalled hearing and seeing people walk past the laneway as she was being assaulted.

After she escaped a number of people who saw Amy, distressed and panicked, came to her aid. The men - police believe a group of three - helped Amy contact a friend and arrange transport home.

“I was not able to comprehend for myself let alone explain to someone else what had happened … so I just went home,” Amy said.

In the immediate period following the attack, Amy found it difficult to focus on her studies and had to eventually leave university.

“Everything seems really difficult … I wasn’t dealing with any of my trauma,” Amy said.

The teen woman, who does not wish to be identified, said the assault has had a serious impact on her life and hopes the reward will help catch her attacker. (Victoria Police)

She tried three times to report to crime before she was able to tell police what had happened six-weeks after she was attacked. She was only recently able to tell her parents.

“It made things for my family difficult as I hadn’t really been telling anyone about it,” she said.

“I didn’t know how to respond to myself … I didn’t know how other people would respond.”

Police have offered up half-a-million reward following a number of appeals for information and the release of CCTV footage and images of the man.

The man is yet to be located.

Although Amy says the process is “slow,” she has thanked police for “working really hard” and says they have made her feel a little safer.

“It was reassuring to have people take it so seriously,” Amy said, adding that the police reaction has reinforced her response to the attack and that what happened was “awful”.

Amy describes her attacker as being aged in his early 20s, around 178cm tall with a lean build, dark brown skim and short hair. He was wearing a dark coloured jacked and long pants when the attack happened.

“I worry mostly that there are people out there who know who this is and have had a similar situation but didn’t report it … it’s probably something he’s done before,” Amy said.

Officers in charge of the Sexual Crimes Squad, Detective Inspector Steve Wilson, said he was hopeful that today’s announcement would lead to someone coming forward with key information.

“This is a horrendous crime that occurred at what is probably the busiest intersection in Melbourne,” he said.

“We know that there were plenty of people around at that time so I’d urge anyone who remembers seeing anything on that night to come forward.

Police have today re-released footage, describing the offender as being African in appearance and was wearing a black puffy jacket with red lining, light blue jeans and white runners.

“It really doesn’t matter how small or insignificant you think that detail might be, it could be just what police need to progress this investigation," Det. Wilson said.

“There is absolutely no doubt that this assault has had an enormous and detrimental impact on this victim’s life.

“She was only an 18-year-old teenager at the time and this attack had significant negative effect on her university studies and day-to-day life.

Detectives also previously released images of three male good Samaritans that assisted the victim at the time of the incident, with one of those men later contacting police.

*The victim’s name has been changed to protect her identity