"We struggled to get a hang on the car in practice and qualifying - it didn't go to plan," Dumbrell said.
"In the races we tried to maximise what we had... but we didn't hit the mark with the car.
"The Triple Eight cars, both in the main series and the Dunlop Series, probably haven't been as quick as what we would have liked this weekend. We've got a lot of work to do."
Despite his disappointing form, Dumbrell's second-placed finish in the last race saw the Red Bull Racing Australia co-driver end the round in third, behind Jack Le Brocq in second.
"Thankfully at the start of race three, the oceans parted and everyone went to the right, so I thought I'd go down the outside," he explained of the final race where he passed eight cars to finish second.
"I just jammed it in first gear hoping it was going to stop so I could pass a few cars and then from there, it was pretty safe.
"I knew I couldn't catch Cam with the pace he had, so I just tried to maximise points."
He and his Eggleston Motorsport crew now set their sights on Winton - where they closed the round in third in 2014 - and plans to take the title fight to Waters as they approach tracks that suit their Commodore.
"Winton last year, we were reasonable. It's a test track that we've done plenty of miles around, so our car should be good there," he said.
"We're going to a couple of tracks now in the V8 Supercars Dunlop Series where we have performed quite well at last year... Queensland Raceway, Townsville, Bathurst - they're more our forte.
"Hopefully we won't let Cam get too much of a lead and hopefully we can zero in on him later in the year."
The Dunlop Series resumes at the Winton SuperSprint, May 15-17.
Waters is currently first in the series standings, ahead of Dumbrell, Le Brocq, Chris Pither and Garry Jacobson.