The fall was witnessed by dozens of onlookers and other boarders.
"The participants were travelling down the hill in groups of four," Mr Wright said. "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."
A 16-year-old who participated in the event told the Herald he had met the 18-year-old earlier in the day.
"I saw it happen ... He just fell off ... No one really knew what had happened at first. We were all pretty shocked," he said. "He landed heavily and rolled. It was pretty sad."
The teenager was wearing a helmet, gloves and safety padding on his knees.
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.
"There was a professional photographer there and he filmed the entire incident. We have got his memory card and the images will be helpful for the coroner," he said.
The photographer and spectators sitting on a nearby grass verge when the teenager died were all being offered help from Victim Support.
The teenager's parents stayed until his body was driven away by an undertaker.
Mr Wright said they had not wanted to leave while their son was still lying on the road. They went 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.
The area is popular with longboarders because of the wide, steep road. They had organised a permit to get the road closed off for the event.
There were medics on site and safety marshalls, including one in a vehicle that followed each group of skaters downhill. The event was stopped as soon as the teenager fell.
The police serious crash unit is investigating the death, which will then be referred to the coroner.
White markers on the road showed where the teenager landed, and Mr Wright said officers would return to the area today to carry out an in-depth analysis of the spot.