Cooper sounded a stern warning that he meant business when he won both the opening two MX1 class races, but then he struck trouble in the third of his five races, crashing his bike.
He fought back through the field to wind up third, behind Taupo's Brad Groombridge (MC2 Racing Suzuki RM-Z450) and Carter, salvaging points that would eventually prove vital later on.
Cooper finished runner-up to Carter in the next race and so it was all-on for the fifth and final confrontation, with just 14 points between them and 55 points offered for a win.
A collision between the two soon after the start saw Cooper tumble from his bike and Carter roar away. Cooper eventually remounted and re-started his bike and the chase was on, Cooper managing to claw his was back to finish fourth, just enough to earn him the outright win.
"I could only do what I could do," shrugged Cooper afterwards. "I took ages to get the bike going because, in the heat of the moment, I was trying to start it in gear. I took quite a while to get going but then I just put my head down and went hard-out.
"It was good that I fought back to fourth position in that race. It doesn't matter in the end how much you win the day by, just that you do enough to get the job done.
"I've got a whole month now before the start of the nationals and the defence of my New Zealand MX1 title. I'll be stronger and faster by then," he warned.
Cooper's junior team-mate Trent Collins, of Cambridge, was impressive also in winning both the 250cc 12-17 years' junior class on Sunday and then backing up to win the Youth MX grade on Monday as well.
In all, Collins (Honda CRF250) won four of the six races he contested and finished runner-up in the other two.
Other class winners were Hamilton's Josiah Natzke (MX2 class, KTM); Tauranga's Jessie Waterhouse (women's class, Yamaha); Motueka's Josh Coppins (veterans 35-44 years, Yamaha); Hamilton's Darryll King (veterans over-45 years, Yamaha); Taupo's Wyatt Chase (junior 16-17 years 125cc, Yamaha); Mangakino's Maximus Purvis (junior 12-15 years 125cc, KTM); Okato's Zak Hetherington (junior 14-16 years 85cc, TM); Matamata's Brodie Connolly (junior 12-13 years 85cc, KTM); Tauranga's Madoc Dixon (junior 8-11 years 85cc); Hamilton's Nick Westgate (pro 65cc class, KTM).