Australia has quinellaed the women's 200 metres freestyle final at the World Aquatics Championships in Japan, with Mollie O'Callaghan edging out Ariarne Titmus in world record time.

In an incredible finish, Titmus led the field and was three-quarters of a second clear at the final turn. But O'Callaghan rocketed off the wall, and as the crowd roared the pair home, O'Callaghan edged clear and touched the wall in 1 minute 52.85 seconds.

Titmus finished in 1:53.01, while Canadian teenager Summer McIntosh came home for the bronze medal in 1:53.65.

It was the fifth time that Australian has done a 1-2 in a final at the world championships.

"I'm absolutely excited. I was not expecting that at all entering this. I just wanted to have fun and just give it a crack. I was really nervous leading up to this because I didn't know how I was going to race," O'Callaghan told Channel Nine.

"The lead-up to this has been so up and down like a roller-coaster, just with injury and then coming off trials and all that. Just to do that, I just think is incredible and I'm excited for what else I can do."

TItmus was disappointed, but gracious in defeat.

"I have to be happy with a PB. I thought I had 1:52 in me if I'm being honest but for anyone to beat me, I'm glad it's Mollie," she told Nine.

In the men's 800 freestyle, Australia's Sam Short produced a big personal best, taking silver in a monumental battle with Tunisia's Ahmed Hafnaoui.

Short, Hafnoui and Germany's Lukas Martens were virtually stroke for stroke for most of the race.

Hafnaoui produced a brilliant final turn, and went on to win gold in 7:37:00, the third fastest time in history.

Short touched the wall in 7:37:76, the fourth fastest time in history and the fastest time ever by an Australian, surpassing Grant Hackett.

American Bobby Finke came home strongly for the bronze.

"I was going for the win. I knew he (Hafnaoui) would come back fast as he did. I got Hacky's Australian record so that's one goal ticked off the list!," Short told Channel Nine after the race.

"It was brutal. My strategy was the same strategy, use the 400 speed to my advantage and try and tire out Bobby as much as I can for the last 50 so he doesn't come back in 25 and come over the top of me.

"I feel like I executed it the best I could today. That shows in my PB."

In the final event of the night, Australia took a silver medal in the mixed 4x100m medley relay behind China.

Kaylee McKeown led out in the backstroke in a time of 58.03, then Zac Stubblety-Cook swam the breaststroke before Matt Temple's butterfly leg left the Aussies two seconds down going into the final 100.

Shayna Jack swam the anchor leg for Australia, and made up a stack of ground, but China's Yujie Cheng touched the wall in 3:38.57 to win his team the world title.

Jack swam a final leg of 51.53 seconds to reduce the margin to 0.46 seconds. Australia took silver, with the United States in the bronze medal position a further 1.16 seconds back.