A 6-year-old Stokes Valley girl died after losing control of her quad bike and hitting a fence, north of Masterton, on Sunday afternoon.
The incident occurred at the venue for the Jayden Kilmister memorial trail bike ride at Te Ore Ore Bideford Rd.
It is believed the girl was not taking part in the memorial event.
Senior Sergeant Jymahl Glassey said the death was "an extreme tragedy" and officers who attended were shaken. "A young girl has hit a fence and suffered fatal injuries." She was wearing a helmet and safety gear at the time and was riding on a gravel driveway, not a track, he said.
St John paramedics, who were on site as part of the event, administered CPR but were unable to resuscitate the girl. She was the sole occupant of the quad bike, Mr Glassey said.