Australia's Mollie O'Callaghan says she was a "wreck" after setting a world record and winning gold in the women's 200 metres freestyle in Fukuoka.

O'Callaghan finished over the top of training partner and Australian teammate Ariarne Titmus to clock one minute and 52.85 seconds in Wednesday night's final at the World Aquatics Championships.

The 19-year-old's time eclipsed the previous record of 1:52.98 set by Italian Federica Pellegrini in the super-suit era in 2009, which was the oldest world record in women's swimming.

Australia leads the medal standings in the swimming program in Fukuoka with six golds following O'Callaghan's victory.

"I was a wreck," O'Callaghan said.

"I kind of looked [at the results board] and I was a bit like, 'Oh, is that me or not?'.

"I am just so proud of myself to do that. It was such an unexpected moment."

O'Callaghan dared not even dream of the achievement, especially given she dislocated a kneecap at training six weeks ago.

"Obviously there's doubts coming with that [injury] … I had a modified program leading in to this," O'Callaghan said.

O'Callaghan overtook Titmus (1:53.01) in the last 20 metres, with the pair having both set world records in Fukuoka.

Titmus broke the world 400m freestyle record in winning gold on Sunday night.

In other events on Wednesday night, Australia won two silver medals through Sam Short (men's 800m freestyle) and its 4x100m mixed medley team.

Short, who won the 400m freestyle gold on the opening night, touched the wall in 7:37.76, bettering Grant Hackett's Australian record 7:38.65 set in 2005.

Tunisia's Ahmed Hafnaoui won the 800m freestyle final in 7:37.00.

AAP/ABC