"The accident is currently the subject of a serious crash investigation by the NZ Police.
"Motorcycling NZ extends its sincere condolences to family and friends," Williams said.
Also known as rider number 912, the teenager attended Waiuku College and was a pupil of Glenbrook School. He suffered a bi-lateral femur fracture in the crash.
Senior Sergeant Amber Stobie said police were investigating the crash on behalf of the coroner.
"The crash occurred on Saturday and the injured rider was transported to Middlemore Hospital by ambulance with leg injuries.
"Sadly, the victim later died in hospital and police were notified on Sunday evening."
The police serious crash unit was also investigating, Stobie said.
Tributes on social media have labelled the death "gut wrenching, wrong, unexplainable, unimaginable and just so bloody devastating".
Nick Adams was said to have been an "amazing friend, brother, son, and had an amazing outlook on life".
A messaged posted by his family on several motocross pages said the teenager was riding his beloved Yamaha YZ125 at the time of the accident.
"Nick's funeral will be held at the Harrisville Motocross track," it said.
"This will be a fantastic celebration of his short life."
The post also invited fellow riders to wear their MX top to the funeral.
Glenbrook School said it was with a heavy heart and great sadness that it announced one of its past students had passed away.
"An absolutely beautiful young person, whose sense of humour, kindness, empathy and cheekiness has been taken too soon," it wrote.
"Our thoughts, love and whanaungatanga are with... the whole family and friends.
"We love you Nick and there are no words to describe the loss your passing brings to us all."
The school had set up a condolence book for students to leave a message or photos to pass onto the family.
Waiuku College also posted a tribute, saying "Condolences and love to the family and friends of Nick Adams who passed away suddenly on the weekend".
"Nick is fondly remembered by his Waiuku College and Glenbrook School friends and community. We have lost one of our special twins," it wrote.