LEIGH SALES, PRESENTER: A lot of parents have to deal with teenagers who go off the rails. If you're lucky, it doesn't last too long and your kid comes out of it OK.

If you're unlucky, it spirals out of control - and the consequences can be devastating.

That's the case for the family of Sydney girl Melissa Dunn. She was a troubled 17-year-old with a drug problem and a track record of petty crime.

In 2012, she was violently arrested. The CCTV footage shows that police used unacceptable force and it raises questions about how officers are trained to deal with teenagers.

As Hayden Cooper reports, Melissa Dunn's story has a tragic ending.

HAYDEN COOPER, REPORTER: George Street, Sydney in the early hours of the morning, January 4, 2012.

A group of teenagers is loitering on the footpath. They've been out celebrating a birthday, but their night is about to end in a violent struggle with police that would leave one of the girls unconscious.

CLAIRE O'NEILL, SOLICITOR, ABORIGINAL LEGAL SERVICE: She was a 16-year-old girl out in the city on a summer's night in January.

The magistrate described the girls that she was with as being a nuisance. And I really think that's as high as it ever got.

JUDY TIMBERY, MOTHER: I was appalled. I couldn't believe it: seeing how he handled my daughter. We're supposed to trust the police - and when you see something like that, who can you trust?

HAYDEN COOPER: This is the spirited face of Melissa Dunn: 17 years old, full of life, and a friend to many.

LEILA ZAROUAL, FRIEND: She was a good person. She cared about a lot of people. She had a good heart. She was always there for you. She was someone you could talk to when you needed someone.

LATAI CARR, FRIEND: She was fun, she was happy. She was very stubborn. She was always down for a good time.

HAYDEN COOPER: But this young Aboriginal girl was often in strife.

Her now empty bedroom reveals a troubled existence. She'd been kicked out of school in year 11. Her parents had split and the girl's constant drug and alcohol abuse with her friends had tormented her single mother.

JUDY TIMBERY: They liked to party, drink, smoke marijuana. She did try the ice: she didn't like it. I knew that she smoked dope. I didn't believe it at first; I was in denial. Then I saw her and that was it. I said, "Don't do it here. Don't do it in front of me. I don't like it."

HAYDEN COOPER: By the time she was 16 her problems had worsened.

JUDY TIMBERY: She was a cutter. She cut her arms a few times. And every weekend after weekend after weekend, I'd be up at the hospital bringing her home.

We went to counselling together: that didn't help.

HAYDEN COOPER: In the group she mixed with, this was nothing unusual.

LEILA ZAROUAL: Nah, she was just like a normal person. Sometimes you have your sad days. She'd come and talk sometimes, like any other person would. But she was just happy. She was always happy, always laughing, always. Yeah.

LATAI CARR: Parties, go to a cousin's house, just... wherever we were at the time, whatever was happening.

We'd drink and smoke bongs. Yeah.

HAYDEN COOPER: That's how events began on January 4, 2012: a group of teenagers at Maroubra Beach in Sydney's east. It was Noeleen Kane's 16th birthday.

NOELEEN KANE, FRIEND: First we had a barbecue and we had a couple of drinks. And then I think we just got a bit intoxicated and wanted to get out of the area. Like, it's a birthday. You know, you're 16. You're, like, drunk.

HAYDEN COOPER: They ended up here, outside McDonalds on George Street, in the middle of Sydney's CBD.

By now, it's 2:00 am.

Captured on CCTV footage, some in the group are still drinking. Noeleen Kane, the birthday girl, in white, arrives and sits down inside. She keeps to herself.

Melissa Dunn, in black, is outside. Briefly she harasses passing pedestrians but otherwise remains with her friends.

Soon the police arrive.

Constable Hugh Michelson recognises Noeleen and calls her over. She ignores him, but then turns back. Melissa joins her.

The policeman issues Noeleen with a "move on" direction. She complies, but again he calls her back because she's swearing at him.

He decides to arrest her.

NOELEEN KANE: I only realised they were there until I was on the ground, arrested.

HAYDEN COOPER: What did you say to the police officer?

NOELEEN KANE: Well there was not much to say when I had a couple of people on top of me.

HAYDEN COOPER: And before that, though?

NOELEEN KANE: Oh, yeah. I didn't know they were there. (laughs)

HAYDEN COOPER: But you were swearing at him?

NOELEEN KANE: Yeah. I did use a couple of bad words.

HAYDEN COOPER: As Noeleen Kane is marched away her friend, Melissa Dunn, follows. In anger, Melissa hits the side of the police car.

Constable Michelson chases and tackles her in the street. She's eventually handcuffed and then dragged in a headlock to the police van.

As the police try to put her inside, she falls, hits her head and is knocked unconscious.

CLAIRE O'NEILL: I think it's very concerning that this particular police officer acted as he did that night.

HAYDEN COOPER: Solicitor Claire O'Neill would later defend Melissa Dunn in court.

CLAIRE O'NEILL: The magistrate described it aptly when he said that what happened, at least at points, was just wrong.

HAYDEN COOPER: By the time an ambulance arrived, Melissa had regained consciousness. She resisted treatment.

Eventually she was taken to hospital, her blood alcohol reading 0.15.

She was charged with hindering and resisting police. Her friend, Noeleen Kane, was released.

(to Noeleen Kane) Do you think you guys deserved the treatment you got that night?

NOELEEN KANE: No. Like, not to the point where I'm on the floor being arrested because I wasn't violent, really. There was no excessive force to be, like, chucked and dragged on the floor. But, yeah.

HAYDEN COOPER: And were you...

NOELEEN KANE: And especially being young, too. Like, you're not really... you don't really know what's going on. Like you're 15, you're a girl. Like, you're pretty harmless.

HAYDEN COOPER: Melissa's mother only saw the footage for the first time seven months later, when the case reached the Children's Court.

JUDY TIMBERY: I broke down. I couldn't believe it. I was just that angry. I didn't get a chance to see it before that.

MICK GOODA, SOCIAL JUSTICE COMMISSIONER: It's pretty shocking. I think any parent in Australia would be very worried if they had their kids treated like that. I think that's my first reaction: how I'd feel if that was my daughter.

HAYDEN COOPER: Aboriginal Social Justice Commissioner Mick Gooda says police need better training at dealing with young and vulnerable teenagers.

MICK GOODA: How do you deal with Aboriginal kids who night get a bit lippy and a bit mouthy? That doesn't justify that sort of treatment that we've just seen.

I'll be contacting the police to have these discussions about: what can we do in a training sense to make sure this sort of stuff doesn't happen?

HAYDEN COOPER: When Melissa got to the Children's Court, the case didn't go well for the police.

Constable Hugh Michelson told the court that Noeleen Kane was aggressive, erratic and unpredictable. The magistrate disagreed.

He said Constable Michelson's action beggars belief, involved an inordinate amount of force and that the girls should not have been arrested at all.

CLAIRE O'NEILL: What the magistrate found was that, in effect, when Melissa hit the side of the van it was the straw that broke the camel's back and the police officer: basically he'd had enough and then he decided to arrest.

HAYDEN COOPER: After an internal police investigation, Constable Hugh Michelson was counselled and retrained in restraint techniques. His arrest of Melissa Dunn and Noeleen Kane was ruled unnecessary. Instead, the review found they should have been issued with court attendance notices.

New South Wales Police say: "The initial investigation and subsequent reviews found no evidence of criminal misconduct in relation to the officer's actions. A coronial inquest into the teenager's death resulted in no adverse findings against police."

Melissa's mother has engaged a lawyer specialising in police complaints to lodge a civil claim.

DAVID PORTER, REDFERN LEGAL CENTRE: The force used was not only unnecessary, but the situation in which the police officer used force was created by himself, failing to use his powers of arrest properly.

HAYDEN COOPER: When Melissa Dunne was found not guilty, she was thrilled.

JUDY TIMBERY: She was that excited for beating them, she went and celebrated with her friends.

HAYDEN COOPER: Melissa's life seemed to be looking up. But three days after the trial, a sudden tragedy: late at night, after an argument with her boyfriend, Melissa Dunn came to this park in Maroubra and hanged herself.

JUDY TIMBERY: I got a phone call from my nephew, Aaron. "Check and see if Melissa's in the bedroom." And I went and checked. I said, "No, she's not here. Why? What's going on?" He said, "I'll call you back."

Five minutes later he called me back. "Come down to the park. It's Melissa. She's dead."

I ran towards the park and I'd seen her laying there in the park.

And then the police just grabbed me and that and took me back over the road.

HAYDEN COOPER: The news of Melissa's death soon reached the Children's Court where, only days earlier, she had appeared.

CLAIRE O'NEILL: It was quite shocking and, indeed, it was quite shocking for the magistrate. I remember him really being in disbelief that it had occurred.

He recalled seeing her after the court case. He'd left court and she was standing out the front of Bidura on Glebe Point Road. And he told me on that Monday that she seemed so happy. She was talking to her friends on the phone and she was with her mother and she just didn't strike you as somebody who would then go that next step of hanging herself.

JUDY TIMBERY: I'm just angry at everyone. Myself, mainly.

It's the first time that Friday night when I was at work. We normally ring each other about two or three times and not one of us rang each other.

HAYDEN COOPER: Several times a week Judy Timbery visits the park where Melissa died. She wonders if the arrest and trial of her daughter contributed to her death.

JUDY TIMBERY: It made me so angry. No phone call from them or anything - not that it's going to bring her back, but...

MICK GOODA: You know, I don't know where... if there is any connection between this incident and that eventually awful event. But, you know, it doesn't help. Those sorts of things don't actually make people feel really good about themselves.

HAYDEN COOPER: Last month Melissa Dunn would have turned 20. Her mother is still shocked by the events leading up to her daughter's death.

JUDY TIMBERY: Sometimes I think she's still here. She's going to walk through the front door. And I know it's not going to happen because I dressed her. I even got her baptised before we buried her.

Just not fair.

LEIGH SALES: Hayden Cooper reporting.

And remember: help is available.

Lifeline's number is 13 11 14. Kids Helpline's number is 1800 551 800.