"Witnesses of the Bourke St attack say they 'froze.' In my scariest moment, so did I."

As a city and a country tries to come to terms with the senseless, violent act of someone who deliberately drove into pedestrians on Bourke Street last Friday, those closest to the incident are asking themselves one resounding question:

Why didn’t I do more?

Five people were killed by the driver of the red Holden Commodore on Friday afternoon, among them a three-month-old boy, a 10-year-old girl, a 33-year-old father and a 22-year-old woman described as a “bright bubble of joy” by those who knew her. An estimated 37 people were injured, with some still in critical condition. In the days since, police have collected dozens of statements, from people who saw a man recklessly driving the vehicle in the vicinity of Bourke Street, and others who witnessed the moments he mounted the street and sped into innocent men, women and children who happened to be there.

Tributes in the form of letters and flowers have mounted in Federation Square; a manifestation of the grief felt by an entire community.

“I’m sorry I was so useless and I froze and didn’t help,” reads one card, alongside a bunch of white flowers.

Another stand-alone card reads, “I wish I could have helped more.”

I wish I could have helped more.

I froze.

I was useless.

These are exceedingly common reactions to witnessing a traumatic event. The bystander effect - the social phenomenon whereby individuals don't offer help to a victim when other people are present - has been an area of study since the 1960s. Psychologists say this happens for several reasons, including uncertainty about the seriousness of the event, perceived differences between the victim and a potential helper, and diffusion of responsibility.

Many of us know about the bystander effect, but we tell ourselves if we ever found ourselves in a crisis situation, we would be different. I always thought I would be the exception to the rule. Freezing in a crisis situation is something that happens to other people - bad people. Not people like me. Not people who are empathetic and considerate and genuinely care about other people and their well being.

But then three years ago, it did happen to me.

On a typical Tuesday afternoon when I was 22, I found myself in a moment of crisis. And I froze.

My twin sister and I were walking down the street, in broad daylight (wearing frumpy tracksuits for those of you who will inevitably wonder in a minute) when we noticed a man walking towards us. We noticed in the same way you notice any small, insignificant details within your day. It's one of those things that would normally fade into obscurity, had it not been for what happened shortly after.

Because as he walked past, he violently grabbed my sister.

My heart stopped.

My sister screamed and fought, as this man held her down and tried to get his hands under her clothes.

I screamed at him. I yelled for him to get off her and stop, but I couldn't move. No part of my body could move. I was completely frozen.

I continued to yell and scream and scan the nearby area for help. I could see at least two people - one at each end of the street. I stared at them. I yelled in their direction. I pleaded with them to help. But they just stood there, stunned.

I looked at my sister, helpless and small lying on the ground while this man had complete control. I didn't know if he had a knife, or a gun, or whether he was going to rape her, or beat her, or take her away. And despite feeling more terrified than I have in my entire life - I just couldn't move.

A few months ago, my sister and I were asked to be on SBS's Insight to discuss our experience. We were among several other people who had found themselves in crisis situations. To be honest, ours felt very insignificant compared to the others.

Watch Peter Davidson reflect on his most chilling moment as a paramedic. Post continues after video...

Alastair Boast was sitting in a tutorial when a fellow student pulled out a gun, and began shooting.

Joel Trist was surfing when he heard a spine-tingling scream from his best friend. He had been attacked by a shark.

Peter Davidson was the first responder for the infamous 1998 Sydney to Hobart.

Lydia Johns-Putra was in Christchurch for a conference when an earthquake hit, and as a urologist, she had the unique skills to help.

From the show, I saw that crisis situations often bring out fortitude and unquestionable strength of character. These people behaved in ways many of us could never imagine. But according to researcher Rachael Sharman, we don't necessarily choose how we respond.

After the show, Sharman assured me that in my situation, I had 'done the right thing.' I don't think she realised how much I needed to hear that. I've always felt weak and ashamed for not being able to move when my sister needed me most - and I think I always will.

When I saw the devastating tributes in Federation Square from those who felt they hadn't done enough, I realised we all have one thing in common - regardless of our response.

We share the experience of having your understanding of the world shatter before your eyes. Of believing that you're safe and secure and nothing can hurt you, and having circumstances tear apart that illusion. We share the nightmares and panic and emotional toll that comes with being entirely terrified for a moment in time - and the change in perspective that comes when you realise you've been profoundly and irreparably changed by it.

No matter how you respond in a moment of crisis, whether you're a bystander or a hero, your world changes. As Rachael Sharman says, perhaps you can't choose how you respond to an emergency.

You are not to blame for freezing when out of nowhere, a man mounted the street and drove his car into innocent people.

You are not to blame for not doing 'enough,' when something happened that you never, ever, ever expected to happen.

You are not 'useless,' for witnessing the death of people who moments earlier were passing you by on the street, and not knowing what to do.

All responses to these situations are equal and have their own value. No one asks to be put in a terrifying situation, and we don't always know the 'right' thing to do.

We're human - and nothing reminds us more of the fallibility and complexity of our condition than being in the throes of crisis.