Robertson fired rounds of 70 and 68 for a three-shot win over Matt McLean, of Harewood.
McLean shot a 68 in the first round to lead but a couple of costly errors late in the second round helped Robertson to clinch the win.
McLean fired a 73 in the second round.
It was the first time Robertson had won the Classic on his home course. A farm worker, Robertson is also the Taieri Lakes strokeplay champion.
Phil Bungard, of Island Park, recorded 144 for third.