Roulston, 29, said he got lucky after taking a gamble near the Ngunguru Reef which gave him an early lead.
"It was [the conditions] pretty messed up," said Roulston. "I got through on a clear spot over the reef, and it wasn't clear for the others.
"I thought everyone was going to go through it and I looked back to see if anyone was coming and no one was. If I had seen no one was coming I probably wouldn't have gone through."
However, Roulston said credit needed to be given to Moana Nui waka makers who supplied him with a specialised waka for the day's conditions. "Different conditions, different canoe," said Roulston. "Without them [Moana Nui] I couldn't do it."
He added Saturday's win was not expected and applauded his competitors for sticking it out in the tough conditions. "I wasn't expecting to win, there were heaps of good paddlers. I thought I might have been up there but it was a good surprise."
Roulston described the Bo Herbert Memorial Race as being one of the top singles races in New Zealand.
Conan Herbert, son of Bo Herbert, said the memorial race was in its fifth edition and had a variety of meanings to different people, but always it was a special day for his family.
"Bo's race means different things for different people," he said. "Some are out there to be as good as Bo and try to win, and others are out there to try and complete the race course. For me and my brothers it's a celebration, but also a competition."