Wightman recovered from an average start and emerged as leader at the end of lap one, until he was overtaken by flying Eketahuna rider Chris Dickson, who pulled a lead of more than a minute and looked to be racing away to victory.
However, a long pit stop cost him a lot of time which gave the lead back to Wightman.
When Wightman failed to conquer a hill shortly afterwards, he handed the lead over to Walker, until a determined Wightman fought back to eventually take the win.
"A second placing would have been good enough for me to earn second overall in the series, but I wanted the win all the same," said Wightman.
Wightman eventually won the race by 12s from Walker, with Dickson the third rider to pass the chequered flag.
Although Wightman was already assured of claiming the No2 ranking for the series, with a solid points lead over Walker at the start of the day, his win on Sunday cemented this.
Wightman's younger brother, Nick, took his KTM85 to win the two-stroke under-85cc class title.
Pahiatua's Paul Whibley won the senior race on Sunday to also claim the senior title overall, finishing the series ahead of Mokau's Adrian Smith, Hamilton's Andrew Charleston, Rotorua's Scott Birch and Maruia's Ethan Bruce.
Another KTM rider, Rotorua's Jane Roberts, was the top senior female.