The 16-year-old Aucklander took a fine win, recovering from a sluggish start that saw him drop from third on the grid to run fourth in the early laps.
The win secures him the second position in the 2011 championship and the rookie title.
Cassidy also took fastest lap as he led home Australian Nick Foster by 1.4 seconds, with Auckland's Alastair Wootten third.
Cassidy, Foster and Wootten diced furiously for the lead throughout the 12 laps after pole winner and early race leader Mitch Evans made a rare mistake and retired near the end of the fourth lap.
Evans started from pole position having qualified nearly half a second faster than anyone else but was immediately under attack from Foster, a fast starting Wootten and Cassidy during the frantic early running.
The pressure told on Evans near the end of the third lap when he misjudged a turn-in line and clipped the corner marker with a front wing. The impact forced him onto the grass and into his first retirement of the 2011 season.
Going into the three final races of the season Evans had finished all 12 races to date this year inside the top three.
