He clocked a personal best of 52.16 seconds – almost half a second quicker than his 100m ‘fly time at the NSW state titles three weeks ago.
Chalmers’ victory came after he trained twice on Sunday before a night final he found surprisingly comfortable.
“It wasn’t actually to painful, to be honest with you,” he said.
“I have been doing so much freestyle that I’m probably pretty fatigued in those muscles at the moment.
“So to change it up … I haven’t done a lot of butterfly, so it was pretty good.”
Chalmers, the 2016 Olympic 100m freestyle champion, swam butterfly as a junior.
“Up until 2014, I did a fair bit of butterfly but I haven’t done hardly anything since then,” he said.
He wasn’t worried about his butterfly effort taking some steam out of his 100m freestyle swims on Monday.
“Nah, not at all,” he said. “I’d like to think I’m in pretty good shape, I’m pretty fit at the moment.