The revised points standings showed Bugden with a 16-point lead over Shirriffs with just one race to go, meaning even if Shirriffs won the final outing, sixth place or better would be enough for Bugden to wrap it up.
Bugden remained calm and raced his way to fifth in race two.
Meanwhile, Shirriffs crashed out when he was running in second spot in the final race. While Shirriffs did recover his bike and carry on, he managed only sixth, the race won by Wellington's Sloan Frost, with Cole the runner-up.
Bugden was New Zealand superbike class champion three times consecutively between 2007 and 2009, and then won it a fourth time last season.
"I was pushing hard at the start of that second race, hoping to create a break on the field, but everyone simply came with me," said the 32-year-old Bugden.
"When I saw that Craig (Shirriffs) had crashed, I backed it off a bit. I'd done the maths and knew what I had to do to win the title."