Monday, 8 April 2019:

Australian Olympic swimmer KYLE CHALMERS poses during a portrait session in Adelaide, Australia.

Freestyler Kyle Chalmers entered a butterfly race for the first time at the Australian swimming championships as "a bit of fun".

Now, he's a national champion in the stroke after winning the 100m title in Adelaide on Sunday night.

"I stuck my head down and tried to swim similar to how I swim freestyle races - come home strong," he told AAP.

"It's a bit more challenging doing butterfly though."

He clocked a personal best of 52.07 seconds - almost half a second quicker than his 100m 'fly time at the NSW state titles three weeks ago.

And 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.

And 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.

"We have practised backing up a lot in training so it will have no effect."

AAP