Australia's McKeown swims second-fastest 100m backstroke of all time

World record-holder Kaylee McKeown ramped up preparations for the defence of her Olympic 100m backstroke crown with the second-fastest swim of all time on Tuesday.

The 22-year-old blitzed home at the Australian Olympic trials in 57.41secs, just shy of her world-record 57.33 and ahead of Mollie O'Callaghan who became only the fourth swimmer ever to dip below 58.

O'Callaghan's 57.88 in Brisbane was enough to also easily qualify, but she may not swim the event in Paris this summer given her likely heavy freestyle and relay schedule.

Should O'Callaghan opt out, American Regan Smith and Canada's Kylie Masse are set to once again be McKeown's main challengers.

"Look I'm not going to lie, I was a bit disappointed with that," said McKeown of her time. She holds world records in all three backstroke disciplines.

"But, you know, there's always room for improvement. Glad to have another swimmer in the 57-club."

McKeown has swum nine of the 10 fastest times in history and will return to the pool on Thursday for the 200m backstroke, where she is also the defending Olympic champion.

She is also scheduled to contest the 100m and 200m freestyle, having already booked her ticket to France by clocking the fastest 200m medley since the 2016 Rio Olympics on Monday.

Despite joining McKeown, Smith and Masse in the sub-58 club, O'Callaghan said she had yet to finalise her Olympic plans.

"To touch in 57.8 is really amazing, I was not expecting that," said the 20-year-old, who is the 100m freestyle world record holder.

"I'll wait and see how the rest of the meet goes. Hopefully I qualify for another two or three individuals and we'll decide from there."

Third-placed Iona Anderson was also under the qualifying time and will step up if O'Callaghan drops out.

Rising star Maximillian Giuliani outswum the more-fancied Elijah Winnington and Thomas Neill to book his ticket in the men's 200m freestyle, hitting the wall in 1:45.83.

But he will face stiff competition in Paris with a host of swimmers going faster this year, led by Germany's Lukas Marten and Britain's Matthew Richards.

Jenna Strauch, on the comeback from a knee injury, was fastest in the women's 100m breaststroke. Her 1:06.90 was outside the qualifying time but she will likely be on the team for the medley relay.

It was a similar scenario in the men's 100m backstroke with Isaac Cooper (53.46) touching first but outside the time required. He too should be on the plane for relay duty.

Mitch Larkin came fourth to miss out on becoming the first Australian man to make a fourth Games. He will get another chance in the 200m backstroke later in the week, an event he won silver in at the 2016 Rio Olympics.