Simone Ackermann came home in fourth, 29 seconds behind the winner, while Chloe Seaman was fifth in 47:25.
The men's race was just as tense, with Drinnan and Sebastien Priscott putting on a clinic in open-water swimming.
In the end it was Drinnan who took out the title in 41:43, with Priscott sitting just two seconds in arrears.
Matheus Ribeiro (42:43) completed the podium, while Bevan Jacobs (42:50) and Carl O'Donnell (42:53) rounded out the top five.
The I'm Going Long discipline wasn't the only one which threw up exciting and tense contests.
In the 1km Step It Up race it was a fight to the finish between the men's and women's winners, Dave English and Hannah Milne.
English eventually took out what was an enthralling race in 12:32, with Milne nine seconds off.
Marc Seaman (12:58) and Alex Dunkley (13:04) were on the men's podium while Kendall Oliver (15:30) and Kate Wills (15:47) grabbed top-three finishes in the women's race.
Meanwhile, the OceanKids 200m race produced some quick times with each of the top three finishing inside the winning time from 2016.
Ella Crowe was the star of the show, taking the title in 3:07 which shaved seven seconds off last year's winning mark.
Liam Shanahan (3:11) and Talitha McEwan (3:12) also beat the mark in impressive efforts.
The Give It A Go event, raced over 500m, was another top competition with Jayden Collins winning comfortably in 6:19.
Paul Estival (7:01) and Anna Lepua (7:14) were the next two quickest through the water.
