Grimsey, who missed out on Olympic qualifying for Australia, had set himself for today's race in preparations for a crack at the record for the English Channel later in the year.
"I came here to win. The $5000 prizemoney was a big lure, for sure, and I targetted this result," Grimsey said.
"I didn't realise Kane had got so far ahead on the first lap until I got to the feed point, and I thought that I may not catch him but I managed to bridge up to Kane and went for it on the last lap."
Radford said he had decided to push hard on the first lap, tactics he would not normally use, but overall was pleased with the outcome.
"I came here to progress my Olympic campaign," he said. "I would have liked to have won but I got the job done. That's what I came here to do. I have four months until Portugal and I am looking forward to getting back into training.
"I think I am on track for London and I am now one step closer to that dream."
While Grimsey and Radford cleared out from the field, Auckland's Jonathan Pullon produced a strong final lap to overtake Wellington's Casey Glover and secure the second qualifying spot for Portugal.
Defending champion Phillip Ryan withdrew on the second lap after a recurrence of his shoulder injury.
Baker made the early pace in the women's race before she and Gorman cleared out, powering through the men's field that had started three minutes ahead.
Gorman, a five-time Australian open water champion, bided her time before powering ahead on the final lap to win in a time of 2:08:13, with Baker two minutes back and Webby third seven minutes behind the winner.
Gorman, who has already qualified for the Olympics, said the experience was ideal preparation for her bid for a medal in London.
"It is conditions we could expect in London so that was good," she said. "I only have one more 10km swim before the Olympics, so this was good racing practice and shows that my training is on track.
"I am still in big training load right now so the result was pleasing."
Baker said her swim was encouraging as she works towards her goal of Olympic qualification.
"I felt really good. The conditions were really tough. It got colder and colder and so I struggled a little on the last lap with the temperature.
"The key was to qualify for Portugal and I did that strongly. I achieved my goal while still in big training so it gives me confidence that I am on track for Portugal and hopefully London."
The championship was part of the Epic Swim, a range of championship and recreational swims that attracted about 500 competitors. The event finishes tomorrow with the national 5km championship and the New Zealand masters championship.
Men: Trent Grimsey (Australia) 1:59:50, 1; Kane Radford (Rotorua, HPC) 2:02:52, 2; Jonathan Pullon (Waterhole, Auckland) 2:08:07, 3; Casey Glover (Capital, Wellington) 2:08:25, 4; Troy Balvert (Waves, Otago) 2:09:05, 5.
Women: Melissa Gorman (AUS) 2:08:13, 1; Cara Baker (Howick Pakuranga) 2:10:40, 2; Charlotte Webby (Aquabladz, Taranaki) 2:15:42, 3; Emily Seymour (AUS) 2:19:59, 4; Grace Sommerville (Ashburton) 2:20:29, 5.
