On countless occasions over the past three years Kathy Kelly has willed herself to tell her terrible story but repetition has done nothing to reduce the power of the awful narrative.

Nor has it diminished her fierce determination to spare others the unimaginable torment into which she and her family have been pitched.

Kathy is the mother of Thomas Kelly, the 18-year-old who died after being felled by a single punch on his first-ever night out in Kings Cross.

On the night that everything changed, Kathy and her husband Ralph were preparing for bed at their Southern Highlands home. Earlier, Thomas had spoken on the phone to his mum.

"I said, 'I just wanted to make sure you're OK and have a lovely time tonight and be careful'," she says. "And he said, 'Yes, mum, I'll be careful'. I said, 'I love you' and he said 'I love you, too, mum'. Those were the last words we spoke to each other."

The phone rang again at 10.25pm but this time it was a doctor from St Vincent's Hospital with the news that Thomas had been attacked and was gravely ill. Kathy and Ralph raced to Sydney, terrified and confused. By 4am they knew the worst – that their boy would never recover.

"I don't even recall how I reacted," Kathy says. "How do you react when somebody tells you your child is not going to make it when he went to go to a party and he was living the dream, out for his first night holding a girl's hand that he was about to ask to be his girlfriend?"

Kathy has shed rivers of tears since that night and, as she describes the moment Thomas' life support was removed, the emotion threatens to burst through again. After a couple of seconds she recovers herself.

"He had to die of cardiac arrest," she says. "Ralph and I and the two [other] children stood there and we watched Thomas' heart stop in front of us and we watched him turn blue and we basically saw him suffocate in front of us.

"Within a few minutes they took him away for surgery for the organ donation. Later that evening we left the children in a hotel in Potts Point and Ralph and I had to go back to the hospital to identify Thomas' body.

"It was such an incredibly difficult thing to do. We were with two tall, strapping policemen and when we were walking out they both had tears pouring down their face. They had children like we have."

It is this desolate story, told with such calm determination by Kathy and Ralph, that has helped chip away at our booze-and-bash culture. There is now mandatory sentencing for one-punch killings. New lockout laws, which have proved controversial in some quarters, have been introduced. The Kellys also fought successfully to have the sentence of Thomas' attacker significantly lengthened and by pure force of will have created a powerful organisation, The Thomas Kelly Youth Foundation that is doing practical work to keep kids safe.

It is this story that Kathy will take to Saturday's TEDx event to spread her message that, as a community, we must face up to our alcohol problem.

"We need to speak out," she says. "We need to be the voice of Thomas. We need to be able to drive the community to do something.

"We know we are not talking about changing our culture in the next couple of years. We're talking about our children's children. We're trying to create behavioural change in our youth so our children's children have got the chance of going out and walking the streets of our cities safely."

Inevitably, the personal cost of living out their nightmare so publicly has been considerable for the family.

But where many other families have shattered in the face of such intense grief, Kathy says she, Ralph and Thomas' siblings, Stuart and Madeleine, have been drawn more tightly together.

"Ralph and I have survived this terrible journey because we have two other children that we love and who need us," she says. "We have often said … that if we didn't have those other kids, then what would be the purpose of us being here? We have lived our lives … I don't care if my life is over. If Ralph and I could have taken his spot in that hospital we would have done it in an instant.

"Ralph and I have a very deep bond that we probably didn't have before we lost Thomas. Nobody understands me more than him and I don't think anyone understands him more than I do."

Kathy can scarcely bring herself to mention the name of Kieran Loveridge, the man who killed Thomas with such a random and senseless act of violence. Loveridge is serving a 14-year jail term, after his original five-year sentence was increased.

"The reason we became so public was that I wanted people to understand what we were going through," Kathy says. "Not so that we had sympathy. I wanted them to understand that if you were put in this position, you would not be hugged at the DPP's office. You are told  from the very beginning that you are not going to get anything. Basically: tough luck. Just sit back, shut up and accept that is what it is."

Kathy says there is a "very, very small part" of her that feels sad for Loveridge and his family.

"[But] I will never forgive him for what he did to our family. I will never forgive him for taking away my son and there hasn't been any signs of remorse.

"As a mother I cannot comprehend that we have never received a note from his family saying we love and we support our son and I hope you understand that we will always care for him but we are so sorry. There has never been anything. How can I forgive people like that?

"That makes it really hard for us as a family because living with that hatred and anger inside you is another burden for us. I don't want that but why should I just be this loving forgiving person when they took everything from us. They took my first baby."

With the story told once more, Kathy visibly relaxes. But she knows she will have to go on baring her pain and telling the story many more times before she and Ralph see the change they know must come.

It is hard to conceive of her not succeeding because no one – not premiers or police chiefs, legislators or lobbyists – can ultimately stand in the way of this dignified woman's resolve that her son will not have died in vain.

Kathy Kelly speaks at Tedx at Macquarie University on Saturday, September 26.