On a course well suited to their talents, Johnstone and Groves had forced the issue and gone clear with two Manawatu riders by the end of the first lap.
With fellow club man Adrian Pettit playing policeman and helpfully slowing the chase group behind, the front four stayed clear of their rivals.
A determined Groves ground out a strong win after attacking the climbs throughout and is clearly over his recent health issues.
Behind theleading quartet, Pettit and fellow Wanganui riders Richard Stanton and Steve Cowan broke clear of the remaining riders on the final lap.
They combined well to come home in fifth, sixth and seventh places respectively - ensuring more points for the local team. Wanganui riders Lewis Bunker, Boaz Darby and Darcy Forrester rounded out the top ten.
In the two lap, 40km C-grade race - which included a big group of U17 girls from the development squad camp Wanganui's Michael Wright sprinted home at the head of the bunch to claim victory after being prominent throughout.
After some long turns on the front, last weekend's winner Allan Luff was the third C-grader behind Manawatu's Michaela Drummond.
Ross McNeill and Dan Richards contributed to Wanganui's tally with fourth and fifth placings respectively in a big line up for the minor spots.
Pat Carroll came in seventh and Martin Hogan out-finished the next bunch home to take eighth.
In the one lap, 20km D-grade event, local rider Paul Darby did well to claim fourth after staying in the lead bunch throughout the race.
An unofficial tally of the day's points saw Manawatu slightly ahead on the day, but Wanganui still narrowly leading the series by virtue of the club's big win in round one.
The competition returns to Halcombe next weekend for round three.
Results
B Grade: 1. Phil Groves (Wanganui); 2. Pat Johnston (Wanganui); 3. Ross Rainham (Manawatu).
C Grade: 1. Michael Wright (Wanganui); 2. Michaela Drummond (Manawatu); 3. Allan Luff (Wanganui).
