Lawn finished in 4:22.54, nearly seven minutes ahead of second-placed Hannah Lawrence (Auckland).
"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," Lawn said.
Lawn did not quite bury all of her ghosts, with a rear wheel issue halting her progress on the bike on Saturday, but once onto the run she was able to put daylight between herself and fellow leader Belinda Harper, a former Hawaii age-group world champion.
"The rear wheel kept locking up 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."
Lawn, a seven-time Ironman New Zealand champion, was the class of the field with the best swim and second fastest on the bike and run splits but said she never took victory for granted, despite the pre-race withdrawals of likely contenders Sam Warriner, Janine Sax and Anna Cleaver.
"These sorts of races you're generally against yourself and the distance anyway until it gets to that point in the race where you're racing each other but I don't own this race. Every time you toe the line it's a clean slate and today I won but next year maybe it's someone else. I like victories where it's a real race, like the battles I've had with Kate Bevilaqua and Anna, where you push it to the limit. It's not that enjoyable at the time but that's the art of racing."
One of the pre-race favourites, local athlete Michelle Bremer, the recent Ironman Western Australia winner, 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.
There were a collection of well-known athletes from other fields who took part in the event including former All Black Ian Jones (4:55.22) and cyclist Gordon McCauley (4:34.40), while triathlete Kris Gemmell completed the run leg as part of the Broom Brothers' team.
Results, Port of Tauranga Half (2km swim 90km bike 21km run):
Elite men: Cameron Brown (Auckland) 3:53.42, 1; Callum Millward (Auckland) 3:54.18, 2; Tim Berkel (Australia) 4:01.28, 3; Shanon Stallard (New Plymouth) 4:06.51, 4; Daniel Plews (Auckland) 4:08.00, 5.
Elite women: Joanna Lawn (Auckland) 4:22.54, 1; Hannah Lawrence (Auckland) 4:29.37, 2; Tamsyn Hayes (Dunedin) 4:30.16, 3; Elaine Brent (Wellington) 4:31.54, 4; Belinda Harper (Auckland) 4:32.16, 5.
Teams: Rehab Physio Centre (Mt Maunganui) 3:45.40, 1; BlueSeventy 3:52.27, 2; Team Brick 3:52.42, 3.