Haumoana man Andrew Garland was on the beach when he was alerted to the mishap when he saw a young girl on a cellphone and the front wheel of the three-wheeler in the air.
Mr Garland said that when he reached the two youngsters the boy was in obvious agony. The youngster had been tossed backwards as it rode up a shingle bank near the river mouth, at least a kilometre from the domain at Haumoana. The front wheel of the trike was in the air, the machine was at risk of toppling on the boy, and the girl appeared to be having difficulty telling emergency services where she was.
Mr Garland's short trip on his own quad bike to get fish and chips became a rescue mission lasting more than an hour before the boy was able to be rushed to hospital by ambulance with suspected pelvic and leg injuries.
The boy is understood to have been a recent arrival in the area, and it was not clear whether he had authorised use of the trike.
He was discharged from hospital after treatment.