Mainstone finished 10 seconds ahead of fellow Aussie George O'Brien (Gold Coast) with the battle of the Bay of Plenty boys Nathan Capp and Kane Radford adding some drama to the third place on the podium.
Rotorua's Radford held off Tauranga's Capp until the last 50 metres when Capp stormed past to take third place in 28.38 with Radford two seconds back.
Radford had been based in Perth but, after a break with his coach, is back home. He was disappointed with his swim, but holds a slender lead from Capp in the overall series standings.
But Capp, the New Zealand 1500m freestyle record holder, was delighted with his podium finish to go with a second place in the earlier Wellington leg.
"I am happy with how I went. It was definitely the strongest field I have swum in so far in this series," Capp said. "It probably was in my favour a little bit being on this side (Pilot Bay), considering the conditions today are so flat. It was almost exactly like swimming in the pool, but my navigation is still not that great and I find I am swerving all over the place."
Beating Radford home for some Bay of Plenty bragging rights was a bonus for Capp. "Racing Kane is always fun. It is always good to get one up on him though."
Women's champion Gorman missed five months last year with a serious shoulder injury, but showed her class to blitz the women's field by 59 seconds in 29.34 ahead of New Plymouth's Charlotte Webby with Leah Cutting (Adelaide) third.
Like Mainstone, the win is Gorman's second of the series, and puts both in pole position to win the series, to claim a cash prize of $10,00. "I'm excited of where I am at heading into the final race. With two wins under my belt it definitely takes the pressure off a little bit."
The final race is the King of the Bays on Auckland's North Shore next month with 200 points up for grabs for the winner.