A police spokeswoman said the death had been referred to the coroner.
Tiaan was the son of a former Ruatoria police sergeant who suffered a serious brain injury in a rugby game in 2008, and who has been wheelchair-bound since.
Kahika was speaking about his grandson's death in the hope other young people "can learn from what's happened".
"There was no need for him to go so young ... I told the young people 'my grandson was 13-years-old. The legacy of his life ended right there. There will never be another Tiaan'.
"If he had lived a bit more, maybe had a child, that's his legacy. There's no hurry to grow up, a lot of them are in a rush to grow up."
On Tiaan's birthday on Saturday, his older three sisters tied balloons to his casket, Kahika said.
His tangi will take place at Waiaua Marae tomorrow afternoon.
The Opotiki College student was being remembered as a talented rugby player and someone who was growing in ability in a new sport - golf.
And for his smile.
"He was a very cheerful boy. He was always smiling and he always wanted to do new things."
But life was not always easy.
In 2008 his father, Hone Herewini, received a serious head injury while playing for the Territorials rugby team.
He spent several days on life support and several years in rehabilitation and residential care.
Herewini, who also served as the sole charge police officer at Te Araroa, near East Cape, remains confined to a wheelchair and has caregivers at home to look after him, Kahika said.
"The first couple of years it hit the family pretty hard. They went through a lot."
Herewini had been told of his son's death, but because of his brain injury sometimes became confused.
"Now and then he thinks it's a joke. And now and then he realises 'my son's gone'."
