This is not a story about what's been lost. It's a story about what's been found.

Twenty years ago on Thursday, Nesan Kistan's father was killed. He also nearly lost his mother.

His parents, Tony and Sarah, were on holiday in Tasmania, visiting Port Arthur in the deep south-east of the state. They were having lunch at the historic penal colony's Broad Arrow Cafe when a man finished eating at an outside table, walked inside, pulled a semi-automatic rifle from a sports bag and murdered 20 people in less than two minutes.

Tony pushed Sarah to safety before being shot dead at close range. By the time Martin Bryant was arrested the next day, the tally was 35 dead and 23 wounded – at the time, the world's worst massacre by a lone gunman.

The moment was life-changing for Nesan Kistan, then a 22-year-old studying a double honours degree at Wollongong University, but not necessarily in ways that could have been expected. His Dad was gone in unfathomable circumstances. Amid the grief, he abandoned his ambition for a secular career with a big pay cheque and went to work for the Salvation Army.

"I just decided that life is so short that every moment had to matter," he says from the home he shares with his wife and four children. "And what mattered most would not be just simply going through the treadmill of life.

"I needed to do something, and the Salvos provided a great platform to do something that was meaningful, that was significant, brought hope to others and allowed me to use the rawness of my suffering and sorrow to help those with their own suffering."

Hundreds of people will be at Port Arthur on Thursday for a commemoration service to remember the victims. Guests include survivors, loved ones and a string of dignitaries, including Malcolm Turnbull and John Howard.

Organisers say the service will be reflective, but also try to look forward – an attempt, in a small way, to help in the healing process. Out of respect to the traumatised, Bryant's name isn't found anywhere at Port Arthur, and won't be heard during the commemoration.

Nesan Kistan doesn't have a problem saying Bryant's name. He says he has forgiven him. Why? "Because just like any person in our world he deserves love, grace and acceptance," he says.

"A person in his right mind doesn't do what he did. It doesn't mean I excuse his actions, it doesn't mean I don't have great sorrow and grief over the loss of my father ... but when Jesus died on the cross of Calvary, he didn't just die for certain people, he died for all of humanity."

Nesan Kistan won't be at Port Arthur on Thursday. Neither will his family. For his mother, it's just too raw. "She saw things you never should see," he says.

The Kistans returned to the site on the first and fifth anniversaries of the shooting, and visited the memorial garden created around the cafe's gutted shell. That was enough.

"We've been asked by many people to come down to the service and we've determined that it's not right for us," he says. "I'd rather go to Port Arthur at a time when we can celebrate life and take our kids to celebrate life.

"We'll remember my father's incredible act of bravery, and celebrate the sacrifice that he's made for his children and for my mum. But the 28th of April will be a time when we won't dwell on the past. We're looking to the future."

Within a year of his father's death, Nesan Kistan was working with at-risk youth as a pastor in an inner-city congregation. He later became a captain, and is the church's multicultural director for the eastern seaboard. It is a family business: his wife Cheryl is a minister at Ryde.

He serves as a chaplain with the NSW police, counselling the injured and traumatised and serving as the minister at officers' weddings. He works with local imams to foster stronger ties with the Islamic community, holds public sessions on suicide awareness and works in programs supporting refugees.

Working in a giving profession has enabled him to return support to people who helped his family in their worst time. More than a decade after the massacre, the Tasmanian coroner who had acted as a contact point and provided some solace for the Kistans was in Sydney grieving the death of his own stepson, a police officer. Nesan Kistan was able to do the same in return.

"You never know when you're going to get to help the person who helped you," he says. "I think the best of humanity is when we learn to help each other through the most difficult challenging times."

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