In a stunning display of riding, Groombridge headed off New Zealand's elite 450cc riders and if he can continue this momentum, he should be a contender to win the MX1 crown.
"I was expecting Hamish to be fast and it's hard to be significantly faster than anyone else on this track or for one rider to completely dominate, so I knew I was in for a fight," said Groombridge.
Dobbyn's performance at the event was also remarkable, especially considering it was his debut ride at a major meeting on the big 450cc Husqvarna and he had been racing a 250cc bike in the MX2 class last season.
Hamish Dobbyn
"We've done nothing to this bike yet ... it's completely stock standard," said Dobbyn, just returned from representing New Zealand at the Motocross of Nations in Latvia.
"We still have a lot of testing to do with the bike but things are looking good.
"I have not spent a lot of time on this bike yet, but I was still hoping to be able to battle for the wins and that's what happened. I had a bit of bad luck in one race and ended up finishing ninth, so that cost me a bit."
Dobbyn was taken down in the first turn in his third of five MX1 class races at Taupo, but his charge through the field from last place to eventually claim ninth spot was an awesome sight.
Brad Groombridge
Meanwhile, it was a similar story in the MX2 (250cc) class where national champion Kayne Lamont was reduced to the role of spectator as he too continues his recovery from injury.
Josiah Natzke (CMR Red Bull KTM) - the national 125cc class champion last season - won the MX2 class overall, finishing the day ahead of former Grand Prix motocross star Josh Coppins, the retired racer who was acting as "fill-in" rider for Yamaha's Lamont.
Cameron Dillon (Huka Honda), Scotty Canham (Patterson-O'Connor Kawasaki), and Campbell King (Bel Ray Pro Rider Suzuki) rounded out the MX2 class top five.
South Island heroine Courtney Duncan took her Fox Yamaha YZ125 to easily win the National 125cc class at Taupo. Duncan finished the day with a 1-3-4-1 score card to beat Onewhero's Jayden Turnwald (MR Motorcycles BikesportNZ.com KTM) and Taupo's Aaron Smith (Epic Decals Yamaha) to the top step of the podium.