Millward was second in his inaugural half ironman on the Gold Coast in October last year and just last week won the Tinman Triathlon in Tauranga.
The 29-year-old said he kept with the top contenders during the swim.
"Cam [Brown] really dropped the hammer at the start of the bike and by 15km had split everyone up," he said. "He is typically a great runner so I figured if we stayed together on the bike it was going to come down to a foot race and that's the way it panned out.
"Cam is a great runner so I wanted to get away from him straight away. He's a wily fox and he's got a lot of experience and I didn't want to be with him over the last 10km of the run. I'm rapt to come away with the win."
Brown said it was nice to go over the ironman course again and get familiar with it even though he knows it well.
"Callum has had a couple of wins so far this season," Brown said. "He's younger so he's got a lot more speed than I have so he should be beating me. I'm very happy to be that close to him."
Warriner beat the old record of 4:34.56 set by Britta Martin in 2008 in winning the women's event.
After exiting the swim in third, the 40-year-old quickly moved into the lead on the cycle and went on to win by a wide margin of three minutes over Gina Crawford in 4:24.37. Candice Hammond was third with defending champion Belinda Harper fourth.
"It was great to come back along the waterfront and everyone just cheering for you," Warriner said. "All I can remember was the glow that I got from winning ironman in New Zealand and that just kept me going.
"I've set no goals for next year. I'm either going for ironman or the Olympics. I haven't decided yet. I'm enjoying not having any goals and just getting out there and racing."
Crawford was in her second race back since giving birth to a son in mid-July.
"My bike is not where it needs to be but I'm happy with the run," he said. "It felt good the whole way."
Last year's winner Terenzo Bozzone competed in the teams' event, covering the 90km cycling section in 2:8.2.
New Zealand women's marathon and road champion Lisa Robertson anchored the Vita Sport team to victory in the teams' race, running the 21.1km in 1:16.56.