The best Myers could do in the heavy conditions was a sixth for one point, while Wheeler made a late charge with a second and seven points to fall two short of Macnab.
MacNab won his first premiership in 2012 and also went into the final race of the 2015 season with an unassailable lead to take top honours. He also has represented New Zealand at polo crosse on three occasions.
His father, Dave, was a multiple winner of the Duke of Gloucester Cup (the Holy Grail for amateurs), as a jockey (Red Lancer) and trainer (Sporting Luca and Jack Of Hearts).
That was Macnab's driving force for Macnab junior to claim his own.
His neice Sarah Macnab, now a professional apprenticed to Myers, won a Duke of Gloucester Cup at her first attempt last year.
"Sarah had one go at it and won, it took me seven years," Scott Macnab said.