Five teenagers allegedly involved in the gang rape of a 13-year-old Australian girl, who killed herself two years later, may never be prosecuted, the state of Victoria's Victims of Crime Commissioner has said.

Cassidy Trevan revealed she had been raped by two boys at a house in a Melbourne suburb, while two girls who had bullied her waited in a nearby room and a third boy stood watch at the front door, in a heartbreaking letter released by her mother after her suicide.

In the letter, Cassidy said she had moved house and gone to a different school but still got “students I’ve never met contacting me Facebook calling me a slut”.

Her mother, Linda Trevan, has campaigned for police to reopen her daughter’s case in the hope of prosecuting the five teenagers allegedly involved.

But, speaking to Australia’s 9News, Greg Davies, the state of Victoria’s Victims of Crime Commissioner, said there was little chance of securing a conviction.

“I would think it would be almost unprecedented to have anyone convicted of a rape years after it had occurred and after the victim had passed away,” he said.

“There may be a very faint possibility that somebody could say ‘we did do it’. But you would need full and freely given admissions from one or more, together with medical evidence that may or may not exist.”

Mr Davies, a former police officer, expressed sympathy for Cassidy’s family.

“I cannot even imagine what they have gone through, it’s just the most horrible situation for a family to be in. Just horrendous,” he said.

“It’s pretty awful to sit back and think there are three males who are quite possibly rapists. And a couple of teenage girls – who are not five-year-olds and don’t know right from wrong – who are alleged to have created the circumstances in which this poor girl has been raped multiple times.

"If they get to walk away from it scot-free, there is very little justice. This is one of the most awful crimes that could happen to a female. And what is their attitude now? One assumes they have shown no remorse for anything they have done.”

Cassidy’s letter said she had been raped by pupils who were still studying at her previous school and that she had been taunted about what happened ever since.

She said she wanted to alert other pupils and their parents to the danger.

“I’m not doing this for revenge to those students that raped me, set up the rape, bullied me about the rape, teased me about the rape or anything like that. I’m also not doing this for attention seeking … I want to make that clear,” Cassidy wrote.

“I’m doing this because over 1,500 students, years 7-12, are currently enrolled at the school and they need to be warned.”

She added: “I can’t stop people from spreading rumours, but at least I could try and spread what really happened seeing as it’s still being talked about anyway. But, as I said, this honestly is more so for students who are unaware of this and are in danger of those students.

“My name is Cassidy Trevan, and I was raped.

“If anyone ever tries this on you, trust me it’s worth fighting! Fight! If you don’t you’ll regret it for the rest of your life like I do. You can do it.

“Be careful. Be warned. Be safe.”

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While she met police officers several times about what happened, Cassidy did not make a formal statement for fear that retaliation by those involved would “push her over the edge”.

She took her life in December 2015.

While the police were told the names of the five teenagers, they were never interviewed, according to Cassidy’s mother.

“All I want is for them to be questioned. For them to not think they got away completely scot-free, no consequences. I want them to feel the fear that Cass felt,” Ms Trevan said.

“A 13-year-old girl can’t be gang raped and then end up suiciding from it and there be no consequences, no-one even questioned.”

She has appealed for the boy who acted as the lookout to come forward.

“I’ve been asked thousands of times to 'name and shame' the five kids, but I'm doing the right thing by not doing that,” Ms Trevan said.