No other vehicles were involved.
The child died at the scene.
Ambulance soon arrived, followed by police and firefighters.
The boy's father was airlifted to hospital by Westpac rescue helicopter in a critical condition. He has since been transferred to Middlemore Hospital in Auckland, where he is being treated for serious burn injuries.
One local woman said that shortly after 4.15pm on Tuesday she "just happened to look out the window and saw all these flames".
She then reports hearing "a big bang" shortly afterwards and then "someone screaming".
A distressed woman at the scene reported driving and seeing the vehicle pass her at high speed and failing to take the bend.
"She said she saw them going really, really fast and going straight over [the stopbank] and hit a tree. She was really upset," the local woman told the Herald.
"It was lucky there were no other cars on the bank or dogwalkers – it's pretty busy along there. It's just horrible."
Locals believe that the child lives near the crash scene.
Police say they are still investigating.