A red-hot Kaylee McKeown swam the fourth fastest women's 400m medley in history Thursday but said she did not plan to race it at the Paris Olympics.

The 22-year-old, who owns world records in all three backstroke disciplines, displayed her versatility at the Australian championships on the Gold Coast to hit the wall in 4min 28.22sec.

It shattered the Australian record set by Stephanie Rice at the 2008 Beijing Olympics and was nearly six seconds quicker than anyone else this year.

Only Canada's Summer McIntosh and Hungary's Katinka Hosszu have gone faster.

It came a day after McKeown became only the fifth woman to dip under 2:07 in the 200m medley, again breaking Rice's long-time Australian record.

But while the 200 is set to be on her Paris schedule, along with the backstroke events, she said the 400 would not despite being a serious medal prospect.

"Nope," she said poolside when asked if she would swim the 400m at Australia's Olympic trials in June. "One and done, that's me."

"I've done it and I'm glad I've done it. It's good to challenge yourself."

In focusing on the 400m medley, McKeown was a no-show in the 100 backstroke despite being Olympic champion.

In her absence, Mollie O'Callaghan, who led home a world-class 100m freestyle final on Wednesday, touched first in a personal best 58.09

Only McKeown and American rival Regan Smith have gone faster this year.

"I would like to go 57, but swimming backstroke out here is very hard," said O'Callaghan, who confirmed she would contest the event at the Olympic trials.

Reigning Olympic champion Zac Stubblety-Cook took out the 200m breaststroke in a race boasting three former world record holders.

He came home by a fingertip in 2:07.50 ahead of Japan's Ippei Watanabe (2:07.62), who was racing on an invitation.

Stubblety-Cook lost his world record to China's Qin Haiyang last year and said he wanted it back.

"Absolutely. You get to the top and realise how good it is and you desperately want to get back there," he said.

In other races, Shayna Jack and Meg Harris clocked a dead heat 24.28 in the women's 50m freestyle, ahead of Emma McKeon, Cate Campbell and O'Callaghan.

Teenager Flynn Southam won a close men's 200m freestyle in 1:46.11 by a whisker from Elijah Winnington, who clocked the world-leading time this season in the 400 free a day earlier.

Lizzie Dekkers posted the second fastest time of the year (2:05.20) in the women's 200m butterfly to signal her Olympic intentions.