He also won this year's FINA open water swimming grand prix circuit, is a three-time Australian open water champion and the current New Zealand open water 10km champion.
In the women's race, New Plymouth's Charlotte Webby was even more dominant. She won by more than two-and-a-half minutes over 17-year-old Aucklander Kirsty Wannan. But Webby said it could have gone better.
"My goggles kept fogging up today, which wasn't ideal," she said. "But in a way I'm pleased these kind of issues came out today, so I can sort them out ahead of the other five races in the series."
Both Webby and Grimsey have earned 100 series points for their wins today, as they chase the $20,000 grand prize for the series winner.
Grimsey confirmed he's entered the remaining five rounds of the State New Zealand Ocean Swim Series in an attempt to win the cash.
The other rounds include Auckland's Harbour Crossing on December 2 as well as races in Wellington, Akaroa, Mount Maunganui and Takapuna next year.