He said the event was well organised and there was first aid at hand. The adults who assisted the teenager tried to get him breathing before the ambulance arrived in what he estimates was up to 10 minutes.
Sergeant Andrew Wright of North Shore police told the NZ Herald the rider wasn't able to negotiate the corner, he "lost control [and] flipped up in the air off his board".
The teen was wearing a helmet, but died almost immediately.
Harris says his friend, who was one of the organisers of the event, had been told by a medic that there was a possibility the teenager had had a minor heart attack ahead of crashing.
"When he tumbled people waited for him to get up but he didn't.
"All the competitors were told to collect their bags and there was one bag left so they got his i.d. out of that."
He said it was the second time the event had been held and it had a beginners', Under 17 and open grade section. Although the boy is understood to be 18, he was competing in an Under 17 heat. Harris said the council had approved the event and the road was closed off.
A local year 12 student involved in organising the event, which had competitors from outside Auckland as well, is in shock after the accident.
"I still can't comprehend that someone lost their life today in front of me," he wrote on Facebook. He said counselling is going to be offered to everyone who saw the incident.
He said the boy who died was a valued member of the New Zealand longboarding scene who had great passion for the sport.
"Sadly we had to lose [him] so early and in such an unexpected way, and I would like to send all my prayers to his family.
"This shows that even in the most controlled situations we must skate within our limits.
"Rest in peace man, your stoke will never be forgotten."
The serious crash unit is investigating and the death will be referred to the Coroner. The teenager has not been officially named.