O'Callaghan stuns Titmus

Mollie O'Callaghan upset Olympic gold medallist Ariarne Titmus with the fastest 200m freestyle of the year Thursday to book a world championship slot from a stacked field at the Australian trials.

Reigning 100m world champion O'Callaghan produced a sizzling final 50m to touch in 1:53.83 ahead of Titmus (1:54.14), who easily won the 400 free on the opening night of the meet in Melbourne on Tuesday.

Both qualified for the world championships in Japan next month and will be hot favourites alongside American great Katie Ledecky and Canadian teenager Summer McIntosh, who previously held the year's quickest time.

"I'm absolutely amazed by that race," said 19-year-old O'Callaghan after a career-best swim over the distance.

"I was so nervous and just hoping for a personal best and try and get top two. I'm just so happy, overjoyed. It's exciting for the worlds and let's see what I can do."

Such was the quality of the field that all the top six -- including Lani Pallister, Maddie Wilson, Kiah Melverton and Brianna Throssell -- went under the automatic qualifying time.

They will be in contention for spots on Australia's formidable world-record holding 4x200m relay team.

Surprisingly, Shayna Jack, who had swum a 1.55 this year, missed the final while backstroke queen Kaylee McKeown was a no-show after her sizzling 100m back performance -- the third fastest in history -- on Wednesday.

Australia's most successful Olympian, Emma McKeon, also opted out, forgoing a 200m relay place to focus on the sprint butterfly and freestyle events.

She instead suited up for the 50 fly but in another upset was squeezed into third behind Alex Perkins (25.92) and Lily Price (25.97).

McKeon won the 100m butterfly earlier this week to book her seat to Japan.

Prodigious young talent Sam Short, 19, put himself in gold medal contention by swimming the world's fastest 800m freestyle this year, by nearly two seconds, hitting the wall in 7:40.39.

Elijah Winnington came second to also qualify.

"That hurt a lot, but I got it and very stoked," said Short. "I held off Elijah in the 400 so I had a lot of confidence. I knew if I was with him at 400 that would give me the best shot."