The fall was witnessed by dozens of onlookers and other boarders.
One of those who saw the tragedy was 15-year-old Harris Keenan who was sitting on the bend where the skateboarder came off.
He told The Aucklander the boarders were travelling down the hill on the lower part of the road, which was closed off in the council approved race.
"There had just been a bit of rain and three of them went into a slide and came off. The last boy came round the corner and seemed to get a real fright when he saw the ones ahead of him and then he came off as well."
Sergeant Andrew Wright of North Shore police told the NZ Herald the skateboarders were travelling down the hill in groups of four.
"The deceased was the last in his group and was going about 40km/h. There is a sharp right-hand corner and the first three skidded around but managed to negotiate the corner. The deceased then lost control of his board, skidded, spun in the air and cartwheeled and then crashed on to his head."
James was wearing a helmet, gloves and safety padding on his knees but died almost instantly.
A medic, who was at the event as required by the organisers' permit, rushed to help him. Mr Wright said the teenager had a weak pulse, but when paramedics arrived eight minutes later, they pronounced him dead.
"All the competitors were told to collect their bags and there was one bag left so they got his i.d. out of that," Harris said.
Once the boy was taken away by an undertaker Mr Wright said the boy's parents had to travel home home to break the news of his death to their two daughters.
"The parents are amazingly composed. They stayed with their son the whole time. They didn't want to view him but they didn't want to leave until he had [left]."
Mr Wright said the teenager appeared to have landed on the back of his head. He was bleeding from his eyes, nose and ears but did not have any facial injuries.
"It's just a tragic accident. There were a couple of scrapes and scratches earlier in the day but this was the only major incident."
Mr Wright said there was no issue with the way the event had been organised or managed. There were medics on site and safety marshalls. The event was stopped as soon as the teenager fell.
A local year 12 student involved in organising the event, which was attended by 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 James 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 police serious crash unit is investigating the death, which will then be referred to the coroner. Police say a professional photographer filmed the entire incident and police will use those images as part of their investigations and that of the coroner.
