World 100m freestyle champion Mollie O'Callaghan edged Olympic gold medallist Emma McKeon by a fingertip in the year's fastest time Saturday ahead of a blockbuster world championships next month.

It was the second time in as many races that the Australian teenager had taken down her more illustrious teammate, having also beaten her in April.

She touched at the Australian trials in Melbourne in 52.48 seconds, with McKeon hitting the wall a fraction behind in 52.52.

Along with Hong Kong's Siobhan Haughey, they will head into the Fukuoka world championships in Japan as hot medal favourites.

"I kind of just had to trust myself and believe that I could touch that wall first," said O'Callaghan, who upset Olympic champion Ariarne Titmus to win the 200m earlier in the week.

"The depth here is incredible, it's so close in each race. I just came into this to just try to get top or relay," added the 19-year-old.

All the top five -- also including Shayna Jack, Meg Harris and Maddie Wilson -- were under the qualifying time.

It puts them in contention for a place on Australia's all-conquering 4x100m relay team, which holds not just the world record but nine of the top 10 times in history.

Cate Campbell, who took bronze behind McKeon at the Tokyo Olympics, is not swimming at the trials, instead focusing on the 2024 Paris Olympics.

Kaylee McKeown, who has been in ominous form in the backstroke at Melbourne, challenging both her 100m and 200m world records, was down to compete in the event but pulled out.

However Titmus, who doesn't usually take on the 100m, suited up and came sixth. Titmus had already booked her seat to Japan with dominant victories in the 400m and 800m freestyle.

World and Olympic champion Zac Stubblety-Cook will also be in Fukuoka, winning his signature 200m breaststroke in 2:07.86.

"It's all about getting the best out of myself and to see how far I can push myself," said the world record holder. "I think I have a lot more to give in the 200 breast. 

"I'm excited for world champs."

Abbey Harkin hit the wall first in the women's 200m breaststroke in 2:23.93, completing a clean sweep having also taken out the 50 and 100. But she was marginally outside the qualifying time.

Australia's top breaststrokers, Chelsea Hodges and Jenna Strauch, are both out injured.

Meanwhile, Bradley Woodward qualified with a 1:56.04 in the men's 200m backstroke while Sam Short made the grade in the 1,500 freestyle (14:46.67).

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