Brown and 2010 winner Michael Poole bridged up to the leading pair by mid-race and put the pressure on over the second 45km lap.
"That was the key for me today,'' Brown said.
"I managed to push things on the bike and put the pressure on Callum, which took some of the sting out of him before the run.''
The German dropped away while Poole cramped early on the run to leave Brown and Millward to duke it out, running side-by-side until the final circuit around the undulating base track of Mount Maunganui.
"Callum is a smart young guy and was running on my shoulder the whole way,'' Brown said. "I was waiting for him to attack but I managed to edge away on a slight downhill. I thought, 'this is it, I have to make it stick' and went for it. I am absolutely thrilled.
"To win this title nine times is very special and to win it again at 39 against the young guys is special. It means a lot. And with eight weeks to go until Ironman New Zealand, it shows I am in really good form, probably more advanced than I have been for some time. That's pretty encouraging, too.''
Millward, who beat Brown at Mount Maunganui last year and at the Taupo Half last month, said it was a tough battle.
"I am disappointed but at the same time pleased,'' Millward said. ``We went neck-and-neck and I gave it my best shot. At 39 years old, Cameron still goes pretty good. I will learn a lot from this for the future.''
Lawn was first out of the 2km swim in Pilot Bay but was rounded up by Auckland's Belinda Harper at the end of the first lap of the bike after suffering from some mechanical issues.
"The rear wheel kept locking up,'' lawn said. ``So I stopped and thought it was fixed but it rubbed a bit more on the second lap. I just had to stay patient, though, and cope with it.''
She came off the 90km bike with Harper, well clear of any chasers but Lawn quickly asserted herself on the final leg to run away for the win in 4:22.54.
"I am very blessed to win a race like this so to win four of them is really special and I am very happy,'' Lawn said. ``Having come off base training, though, it hurt, because the body was not used to working at this level of intensity.''
And the most satisfying aspect of the race for the champion?
"The finish line, of course. I will now step into the preparation for Ironman New Zealand with a good win under my belt, knowing I am fit and injury-free. That gives me confidence.''
Harper faded on the run with Auckland's Hannah Lawrence moving through the field for a career-best second place on the back of the fastest run split of 1:28.23 for the half marathon. Dunedin's Tamsyn Hayes was third.
One of the pre-race favourites, local athlete Michelle Bremer who recently won Ironman Western Australia, lost 15 minutes with a puncture early on in the bike but recovered to finish sixth.
The leading men's team was the local Rehab Physio Centre in 3:45.40 with mixed team winners BlueSeventy second fastest in 3:52.27 from Team Brick in 3:52.42.