The national titles feature the country's elite, but have no bearing on Olympic selection, with the trials for the Paris Games to be held in early June in Brisbane.

With the final featuring five of the fastest ever 100m swimmers, former world champion O'Callaghan powered home in 52.27 seconds ahead of Meg Harris in 52.59, with Australia's most decorated Olympian McKeon third in 53.09.

While the time put her easily inside the Australian qualifying mark for Paris of 53.61, O'Callaghan was disappointed.

Shayna Jack, in fourth, was also inside the required time.

"It's a pretty good time ... I would have liked faster," said 20-year-old O'Callaghan, whose cap slipped in the second lap.

"But there's plenty more things I've got to improve."

Three-time Olympian Bronte Campbell was forced to withdraw from Wednesday's final with a calf injury.

The 29-year-old, who took a break from swimming after the Tokyo Olympics in 2021, had qualified eighth-fastest in the morning heats.

Kaylee McKeown sent a strong message to international rivals about her Paris intentions in the 200m individual medley after skipping the event in Tokyo by smashing Stephanie Rice's long-standing Australian record.

The 200m backstroke world record-holder touched the wall in 2:06.99 to best Rice's mark of 2:07.03, set in 2009, with both swimmers coached by Michael Bohl.

McKeown said she wanted to do the backstroke and IM double at the Games.

"It feels pretty good to dip under 2:07," the 22-year-old told Channel Nine.

"I'm absolutely up for the challenge (of swimming both) ... it will be hard, but I feel like I'm putting myself in a good position to test myself and do the best I can."

In other results, Elijah Winnington (3:41.41) and Sam Short (3:41.64) both posted blistering times in the men's 400m freestyle to finish well ahead of reigning world champion, Korean Kim Woomin (3:45.12).

Zac Stubblety-Cook defended his national title in the men's 100m breaststroke, with the reigning Olympic 200m champion the only swimmer to go under one minute.

The 25-year-old Queenslander was surprised to go 59.85 given he's coming off a three-week altitude training camp in Arizona.

Kyle Chalmers set a personal best time of 23.10 seconds en route to victory in the men's 50m butterfly, signalling he is building nicely towards his third Olympics.

The women's 800m freestyle was a two-horse race between two-time Olympic gold medallist Ariarne Titmus and Lani Pallister.

Titmus, who won silver in the event in Tokyo behind American legend Katie Ledecky, clocked 8:17.08 to defend her national title, with Pallister less than two seconds behind.

Australian Associated Press