After five races on rolling Kuratau farmland, near Turangi, Palmerston's Duncan (13) edged out Brighton rider King (15) for second by three points at the weekend.
Riding in the junior 13-16 years, 85cc class, they both had a fall in the first race's last lap, making the fight for placings "real close", King's father, Trevor King, said.
He added that the "South Island dominated" as Rangiora's Micah McGoldrick was the victor by 10 points on a circuit characterised by its long 1km "dead fast" straight, off camber corners and big bumps.
Absent were the high-flying jumps or short technical series of mounds that punctate the young duo's motocross races.
King said his son, who attends Kings High School, did not have the right gearing to maintain top speed down the straight but made up for it in the corners with his late braking.
Duncan, an East Otago High School pupil, said she had to change a socket on her motorbike to improve her swiftness.
The talented pair will be joined by Cromwell's Samuel O'Callaghan (16) in Taupo this week for final training sessions before the squad is chosen to compete at the world junior motocross championships.
King's chances of being one of the final 30 riders - who will then attempt to qualify for the August 15-16 competition - were "looking good," his father said.
Duncan was more guarded, saying she was "half and half" confident of making the final cut.
Taupo Motorcycle Club president Geoff Hall said 120 junior riders were expected to compete.
