"The pool is beautiful to swim in and I felt really good," he said.
"It was also good to send a message in the heats that we're here to play and I still think we've got a bit more to give as well."
McMillan also took more than 1.5s off former New Zealand captain Glenn Anderson's 100m manikin tow time set at the last world championships in Egypt two years ago. He is qualifying fourth for the final, although he is confident he can pick up more time.
"A couple of things didn't quite go to plan ... but luckily I came home pretty strong," he said.
The New Zealand women's 4 x 50m obstacle relay team - Natasha Hind, Sam Lee, Devon Halligan and Laura Quilter - capped a solid session by leading qualifiers in the heats, while the men's team were second-fastest. Hind and Lee also made the final of the 200m obstacles.
At the master's ocean competition at Christies Beach at Adelaide, Mairangi Bay paddler Travis Mitchell took out the men's 30-35 ski race.