NINE years ago, Michael Piacentini was driving to work over the West Gate Bridge in Melbourne when he saw a white four-wheel drive slow down and pull into the emergency lane.

The peak-hour traffic was moving slowly and Mr Piacentini noticed that there were children in the back of the car. He presumed the man had pulled over as one of the children was feeling sick, or had to go to the toilet.

“I noticed he got out of the car, and I noticed a very large sweat patch on the rear of his shirt, which kind of rang a bit of an alarm bell, I thought: ‘Ah, what’s going on there? He must be a little bit flustered or something,’” recalls Mr Piacentini.

Still driving slowly, he noticed the man pull a little girl out of the car and move towards the fence at the side of the bridge.

At that stage, Mr Piacentini says he “still didn’t think anything sinister was happening”. So he kept driving along in the slow peak hour traffic.

As he moved past the parked car, he changed lanes and looked into his side mirror. What he saw magnified in that rear vision mirror has stayed with him since that day.

“That’s when I saw that monster throw his child over the hand rail.”

The day Arthur Freeman dropped his four-year-old daughter Darcey 58m into the water below the West Gate Bridge has been etched into the memory of many who heard about the shocking story in the news.

The story of a man so filled with rage at his ex-wife and the custody situation with their children that he threw his little girl over the bridge to demonstrate his anger and frustration.

But the people who witnessed the event first-hand have replayed the horror over in their head almost every day since it happened.

Mr Piacentini appears on Tuesday evening’s episode of Insight , which focuses on people who witnessed violent crimes.

He says: “It’s something that I’ll never forget, the way he did it, the way he held the child under the arm ... if you’re going to lift a child up under the arms, the child will tend to try and put a foot on the rail or grab a foot hold … she actually went to do that, as if he was going to show her the view … and he actually just tossed her right over the top rail.”

Mr Piacentini says it took a few moments for him to register what he had just seen.

He started doubting what he saw and began running it over again and again in his mind.

“A father’s duty is to protect his children with his whole being and if necessary his life … for me to think a monster could throw his own child off the top of the West Gate Bridge … it doesn’t compute.”

He pulled his car over, jumped out and started running back towards the white four-wheel drive.

He remembers feeling furious.

“My initial reaction was to throw that gentleman over the bridge,” he recalls.

Because of high winds on the top of the bridge he was having difficulty being heard by emergency services on his mobile phone. So he ran back to his car to speak to the dispatcher.

After ensuring that the water police were on their way, he spoke with other witnesses, who had been screaming at the man to stop. Darcey’s two siblings were in the back of the car, and they were trying to stop him from harming them.

Mr Piacentini says his “20 years working in security” kicked in, and he immediately wrote down every detail he could remember, in case he was required to recount the details to police.

It was an intelligent move. His diary notes were later used as evidence in Arthur Freeman’s murder trial — despite being pulled from the water and resuscitated by emergency workers, Darcey died in hospital later that day in her mother’s arms.

Mr Piacentini says that every time he has to relive the events, the same anxiety and heart palpitations return.

For years he felt “paranoid” every time he drove over the West Gate Bridge — now he simply remembers the little girl, and says a silent prayer.

“Life changed for me forever that day ... it showed that evil exists,” he says.

His partner, Terezia Kalkbrenner, believes what Michael witnessed “has impacted how he sees life”. “He’s not looking for things to go wrong necessarily, but he’s watchful and on the lookout,” she said.

Nine years on, after countless sleepless nights and what were undoubtably symptoms of post traumatic stress disorder, Mr Piacentini says he is “humbled by the whole thing”.

“I’m a little bit more quiet, a little bit more reserved and I tend to watch things a lot more,” he said.

“It’s something I will carry for the rest of my life. I’ll carry it until the day I die”.

Insight airs on SBS at 8.30pm on Tuesday.