Otago Boys' High School pupil Sam Cuthbertson (14) chased Dylan Walsh, of Rangiora, hard in the 13-16 years' 85cc/150cc class but wound up second by 10 points.
Out of the six races, Cuthbertson finished second three times before storming to victory on his Honda CRF 150 in the last final two outings.
Event host South Canterbury Motorcycle Club vice-president Dan Batchelor said he rode "exceptionally well."
"Dylan rides on the ragged edge but Sam Cuthbertson is very, very smooth and controlled."
Sam's father Grant credited his son with improving dramatically recently.
"Six months ago he was 20 or 30 seconds behind him [Dylan Walsh] in a race but he's put in a lot of effort to get faster."
While Jesse Te Kani, of Tuakau, won the 8-10 years' section, ahead of Tyler Darby, from New South Wales, Grason Veitch, of Dunedin, shone in his first junior nationals, achieving third place.
His father Dean said Grason (9) was "very happy" with the result.
Although he had one crash on his KTM 85cc during the weekend, "it wasn't a big one".
"The nerves were there at the start of the event but they disappeared."
Grason was one of the KTM riders who occupied 12 of the 18 podium positions in the six categoies.
KTM riders won 32 of 36 races staged over the three days.
The event's top rider was Micah McGoldrick, of Amberley, who took his KTM 250 to a stunning six wins from six starts in the 14-16 years' 250cc class.
