Thinking the last thing that was needed was another spectator, Duncan avoided the crowd and went home.
It wasn't until he later got a call from his wife in hospital that he found out the accident involved his family. His younger son was dead, and his wife and older son were injured.
Emma, Jacob and Nayan were walking home from the mall when an out-of-control car mounted the pavement and struck them.
"I really struggled to remember stuff for a while. It was the shock and disbelief, just a sense of being lost and empty," Duncan says.
While trying to grieve for Nayan and support Emma and Jacob, Duncan was also trying to help police with their inquiries and fend off journalists.
"As time went on, I just got really, really dark and low, and quite introverted. I wouldn't say I was suicidal, but I didn't have the desire to live. Things like getting out of bed were so damn hard."
"I wasn't looking for company, but there were a couple of people who were really strong supports for me. They would just come around. In hindsight, that was good."
The 2011 earthquake was a catalyst for Duncan to recognise he needed to find a way to be happy again.
Duncan was always an active person, but following the accident he stepped up the running. He saw Mal Law on TV talking about the 7in7 – seven great walks in seven days for charity.
Taking part gave Duncan a real boost so, when Mal's High Five-0 Challenge to run 50 marathons on 50 peaks on 50 consecutive days was created, he eagerly signed up again and wanted to involve his students at Ao Tawhiti school.
Mal suggested he take part on Day 31, Mt Isobel, and Duncan agreed. Later, he realised Day 31 was 9 March 2015: Nayan's birthday.
Duncan's experience with grief and loss, and seeing how the earthquakes impacted the mental health of those around him, meant supporting the Mental Health Foundation was easy.
"It just became an organisation that resonated with me."
Duncan, his family and students and parents from the school have raised nearly $5,000 through the High Five-0, an outstanding effort!