Her younger sister, Greta, also had a strong regatta, finishing ninth overall.
"It's pretty unbelievable, I wasn't really favoured to win," said Stewart, a member of the Wakatere Boating Club.
"You can just spend hours looking at all the names on the trophy so it's pretty amazing to have my name in there as well."
Stewart's victory was hard fought, with just one point separating the top three sailors heading into the final race of the regatta.
Needing to finish ahead of Tauranga's Cole Rippey, with whom she was tied on equal points, Stewart rounded the bottom mark for the final time in third place, with Rippey in second. The Takapuna Grammar student split off to the right side of the course down the last beat and managed to find better pressure than her rival to win the regatta.
"Dad told me afterwards it was just painful watching because it was so close," she said.
The daughter of Team New Zealand logistics manager and former round-the-world yachtie Ian, Stewart has been sailing for only four years but she is already on the radar of Yachting New Zealand's youth selectors.
The 15-year-old has already moved on to the two-handed 420 class, pairing up with another promising young 420 sailor, Eliza Wilkinson, in several regattas over the summer and hopes to eventually step up to the 49erFX, a new class of women's high-performance skiff which will debut at the 2016 Olympics.
Ian Stewart initially worried that a busy summer sailing schedule might put his daughter off the sport, but so far it doesn't appear to have deterred her any.
"I dropped her off at the airport on Boxing Day for the Aussie 420 nationals and she had one day off before getting back into the P-Class and I thought, 'Oh no, I've overcooked this - it's going to blow up in my face'," he laughed.
"Thank God it doesn't seem to have put her off yet."
Ian believes that the number of New Zealand women doing well on the world stage is encouraging more teenage girls into the sport.
"You've got Jo Aleh and Polly Powrie, who are world champions in the 470, and Alex Maloney and Molly Meech in the 49erFX so there's never been a better time for young female role models in the sport."