Since moving to the UK on a British Council creative grant 18 months ago the 32-year-old from Mt Eliza had had his share of ups and downs. His godmother had died of cancer in Australia and he had witnessed at close-hand the terror of the London riots. But today things were looking up. Business was booming, he had employed two people to cope and he had just won the largest contract of his career.

Then everything changed.

At the front of the bus three young hoods hassled a young, pregnant mother with a pusher. She had just got on and they wouldn’t let her pass. When she got off at the next stop three elderly women got on. The youths menaced them, too, and no one, including the bus driver, seemed to be coming to their defence.

Smits decided to act. ''Excuse me mate,'' he called out. ''Do not speak to them like that. Let the women through. How dare you. Would you speak to your own grandmother like that?''

Within seconds the ringleader of the group, 19-year-old Sanchez Brown, was leaning into his face, frothing invective and stuck in an escalating, circular and unwinnable interrogation: ''Who the f--k are you? Shut the f--k up!'' Smits attempted to diffuse the situation, trying to talk reasonably with Brown but it was hopeless. He let out a frustrated, dismissive sigh.

The first blow hit Smits left temple, causing his head to hit the window, cutting his brow open and knocking him unconscious for a second or two. But the punches kept coming. They rained down on him until he realised he had to do something to make them stop. Smits rose out of his seat, surprising Brown for a moment with his six-foot-six frame. Falling on top of his attacker, he was able to pin-down his arms. He was worried the teenager was carrying a weapon in his pocket. Suddenly, someone was pulling Tim Smits away, allowing Brown to get his hands free. He pulled out a knife. ''Then I felt the weirdest thing,'' says Smits. ''He’d been punching me really hard and then I felt this soft 'pat' to the left side of my stomach.'' A red patch spread across his sky-blue shirt.

Incredulous, adrenalin pumping, Smits backed away. ''I was really scared. I didn’t know what to do. I’d never been in that situation before.'' But Brown came at him again, stabbing him in the back top-half of his left leg, piercing the hamstring. The last thing Smits remembers were the screams of the passengers on the bus. He went into shock.

Everyone has a story about confrontation or violence in public. Whether it’s anti-social behaviour on the tram or stumbling across a city street punch-up, witnessing it can be confronting. Deciding to act, to intervene in the lives of complete strangers, though, can be fatal.

In 2007, 43-year-old father of three Brendan Keilar was shot and killed by Christopher Wayne Hudson, after the city solicitor came to the aid of a woman involved in an altercation on the corner of William Street and Flinders Lane. Paul de Waard, a Dutch backpacker who also came to the woman’s aid, was shot and critically injured but survived.

All too often heroes and good Samaritans pay the ultimate price.

The message from authorities is clear: call triple zero and get the police there as quickly as you can. ''Our general advice is always safety first,'' says Victoria Police’s Inspector Ian Geddes. ''And that’s the advice we even provide to our police members ... for someone who doesn’t have that training or skill to intervene – it may just make the situation worse.''

Witnesses to public violence can use their voice to yell out and let offenders know the police have been called and are on their way, says Inspector Geddes, but only if they feel safe to do so. To diffuse a situation on public transport it may be useful for passengers to simply remove themselves en masse.

While the inspector’s advice makes perfect sense, in the heat of the moment instinct can take over. I remember in 2003, watching a man about to hit another man in a busy city laneway. The incident appeared to be a minor bout of road rage. Without thinking of the dangers I instinctively put myself between them, yelling at the top of my voice for them to stop. My volume – perhaps accompanied by the fact that I was wearing a hat and looked vaguely ridiculous – allowed the man about to be punched to run away. I remember adrenalin pumping through my body immediately afterwards. A nearby cafe gave me a stiff drink. For years I thought I did the right thing but today I’m not so sure. I have three children now.

But then don’t we all want to live in a society where people care about each other? Where people do act when they see others in trouble?

''I did a good thing that day,'' says Tim Smits of his actions on the London bus. ''I’m very proud of what I did and I would do it again, in a flash, regardless of my experience and just how much it messed up my life.''

Immediately after Smits was stabbed his attackers kicked down the bus’s emergency door and fled. Fortunately for Smits the bus had stopped outside a fire station and paramedics were on the scene quickly. At Smits’ request Heidi, a fellow passenger, called his older brother Brad and then his parents, Peter and Robyn, who were four days into a trip to central Australia. They were standing next to Uluru when the call came through.

He spent the next week in hospital recovering from a fractured jaw, two stab wounds and serious concussion. Four of his teeth had been fractured and they still give him grief today.

It took police eight weeks to catch his attackers (CCTV was broken on the bus on the day of the attack). The lead attacker, Sanchez Brown, had been videotaped carrying out another assault at a train station. Smits, Heidi and a couple of other witnesses were asked to identify him in a computer screen police line-up. ''I was really worried that I might not recognise him,'' says Smits. ''That was a really hard moment for me. But straight away, the first thing I saw were the eyes… (I remember) the aggression in his eyes.''

For six months Tim Smits threw himself into a routine of physical rehabilitation: swimming, acupuncture and Pilates. He decided to take a year out of his career to focus on getting his body right again but there was a problem. At the time of his attack he only had six months left on his visa. He applied  for compassionate leave to remain but it was denied. He appealed. Meanwhile, he prepared himself to face his attackers in court. More than once the hearings were postponed, adding to his stress but when the day finally came confronting his attackers was difficult. ''I was really conflicted. I was glad we were at this point, glad that they’ve been caught, glad that they’re going to get punished here but at the same time I’m looking at them thinking, ‘I can’t believe this is happening. I can’t believe these guys have thrown their lives away and I’m involved in it’. I felt really bad for them.''

Brown was eventually jailed ''indefinitely'' for a minimum of five years and nine months.

Local news stories applauded Smits’ costly stand against thugs. He received free tickets to the London Olympics and two awards for his bravery, including the prestigious Andrew Carnegie Hero Fund Trust, which included some financial support. So when the UK Border Agency (UKBA) denied his appeal for compassionate leave and attempted to deport him there was a huge media outcry: brave Aussie loses visa bid.

Meanwhile, Smits’ emotional life was falling apart. His relationship with his girlfriend ended; he descended into a spiral of depression, anxiety and post-traumatic stress disorder.

Dr Alexander McFarlane, director of the Centre for Traumatic Stress Studies at the University of Adelaide, is concerned by recent studies that show victims of personal violence, such as Tim Smits, are becoming an emerging group in society. ''What it very consistently shows us is personal violence, from a psychological perspective, is more damaging than other sorts of traumas, like motor vehicle accidents and natural disasters. And I think it’s an issue that the community really has to confront.''

The professor of psychiatry says victims of trauma generally behave in three distinct ways: some turn inwards and withdraw; others, such as Smits, display ''counter-phobic'' responses, showing compassion for their attackers and meet their experience head-on; more troubling, though, is the third group, who might deal with their fear by becoming violent themselves. ''It’s one of the things that you see in soldiers who are confronted with violence when they come back from being at war,'' says McFarlane. ''If they are provoked in an assault they can become extremely aggressive and the reason is because the immediate threat conveys the sorts of threats they had to deal with in the combat environment, when you don’t have the same sort of prohibitions on extreme aggression.''

Nearly three years after the stabbing Tim Smits is not completely out of the woods. But there are a few things going right for him. Spooked by the media coverage, perhaps, the UKBA granted him the six months on his visa that were left at the time of his attack. Even better, in recent months a publishing company has sponsored him as an employee, which allowed him to return to Melbourne for the first time in nearly three years to catch up with friends and family.

He’s become passionately involved with organisations in the UK that divert young offenders from violence through art and other programs such as Art Against Knives and Saracens. He’s talked about his experience on radio and at schools in an effort to break the cycle of violence. He’s forgiven his attackers.

''So many positive things have happened just in the last seven months,'' he says, ''and the people that I’ve met – from the Carnegie Hero Trust, to Art Against Knives, to Saracens, to other victims of crime, even to young offenders who have touched me in how they’ve turned their lives around – all of that stuff is so powerful and it’s gone towards me feeling hugely thankful that this experience happened.

''I know it must sound crazy hearing that but I feel that it shaped me into a much more understanding, grounded, caring person.”

The complex question of whether or not to step in and intervene when you see someone in trouble on the bus or the tram or the train remains just that - complex. At one level Smits wishes people had backed him up on the bus that day in September, although he fully understands why they didn’t. But now he wants people to step in at another level, at a societal level, to intervene in the wider problem of youth violence.

''That’s the way that I feel the community can help, by not just leaving it up to one person to do something about it. We all need to start helping from a grass roots level, from the moment the situation happens to being involved in active discussions afterwards about how can we stop this from happening again? How can we change this?''