The 19-year-old swimmer clocked an exceptional 52.63/100th to beat a high-level field.

Shanya Jack (52.64) hit the wall in second place ahead of seven-time Tokyo Olympic medallist McKeon (53.22) in a race that also featured the experienced Cate Campbell (30), back home.

Campbell, who has won eight medals in four Olympic Games, including four gold, was considering retiring after Tokyo, where she won bronze in the 100m behind McKeon.

But the native of Blantyre (Malawi) made her comeback at this first major event, finishing fifth in 53.78, with her sights set on the World Championships in Fukuoka (Japan) this year and her fifth Olympic Games, in Paris, in 2024.

"I'm so happy to have a great training partner in Shanya. It's working very well," said O'Callaghan, who clocked a time faster than the 52.67 that won her gold at last year's Worlds in Budapest, an event McKeon did not compete.

The Australian qualifying events for the next Worlds will take place in Melbourne in June.

Ariarne Titmus, whose 400m freestyle world record was broken by Canada's Summer McIntosh last month, won the 800m freestyle in 8:20.19/100 ahead of world champion Lani Pallister.

In the other events of these Australian Championships, Zac Stubblety-Cook, world record holder in the 200m breaststroke, won the 100m in 1 min 00 sec 07/100th, while the young Samuel Short, 19, surprised the world champion Elijah Winnington by winning the 400m freestyle, the best performance of the year to the key. His 3 min 42 sec 46/100th is also the 10th fastest time in history.

"Obviously, I'm next to the world champion, so I really tried to fight with him," Short said. "I didn't expect my personal best to be close to two seconds."

© 2023 AFP