Groombridge eventually fought through to close to within two minutes of Whibley, but a runner-up finish was the best he could hope for and he backed off the pace over the final lap.
Charles Alabaster also overcame a difficult start to claw his way up and claim the third podium spot.
Dickey settled for fourth overall, albeit finishing just a few seconds behind Alabaster.
Whanganui's contender Seth Reardon (Yamaha YZ250 FX) came in just over 32 seconds later in sixth spot, beaten to the line by Rotorua's Ethan Harris in fifth.
On his 'home' track, Reardon would have hoped for better as the 2019 season points runnerup, however better placings in the last three rounds could see him possibly discard the Marton result – as riders count their best three finishes for their season points tally.
"It's all gone according to plan. There was a really good turn-out here from both junior and senior riders," said Motorcycling NZ cross-country commissioner Chris Smyth.
There were 22 entries in the junior grade and 51 in the seniors, the high-speed rush into turn one an impressive sight for both races at Marton on Sunday.
"The track here was awesome, with a bit of everything ... fast stuff, a bit of tight stuff, with pine trees and even some mud out there, believe it or not," said Smyth.
"The depth of talent is very good and any rider finishing here today in the top 10 have a good chance of winning the series outright."
Round 2 of the championships is set for Flemington, in Central Hawke's Bay, on March 22, before heading to the South Island for Round 3, near Nelson on April 26, and then it all wraps up at Taupo on May 23.
The NZ Cross-country Championships series is supported by Yamaha NZ, O'Neal apparel, The Dirt Guide, Oakley goggles, Bel Ray oils and Michelin tyres.