"Grason [Veitch] was catching me late in the race and I made a few mistakes, but I kept it cool and won it in the end," said Campbell.
For 14-year-old Dunedin rider Veitch, finishing runner-up in the King of the Mountain feature race was perhaps a bitter pill to swallow after he had dominated the 14-16 years' 250cc class with a hat-trick of wins earlier in the day, making him clear favourite especially being on the more-powerful 250cc machine.
But a rare mistake late in the race caused him to crash while he was closing in on Campbell and so he was forced to settle for the No2 spot.
Matamata's Connolly had also been dominant earlier in the day, winning the 12-14 years' 125cc class with three wins from three starts despite being on a 85cc bike.
KTM riders won five of the seven junior classes that were run.
With wild weather arriving overnight to drench the track, Sunday's senior racing was reduced to just one race per class and the senior King of the Mountain feature race was abandoned altogether.