Australia's Mollie O'Callaghan said her battling qualities clinched her the gold in an "amazing" women's Olympic 200m freestyle race against world record-holder Ariarne Titmus on Monday.

O'Callaghan won in a new Olympic record time of 1min 53.27secs, with compatriot Titmus 0.54 seconds behind and Hong Kong's Siobhan Haughey taking the bronze.

"You've got to accept the challenge at the end of the day," O'Callaghan said. "You can either run or fight. I chose to fight."

"I'm always striving for more and I always put a lot of pressure on myself. My expectations are very high. That was an amazing race. I'm always wanting that little bit more," she said before paying tribute to her team-mate turned rival.

"It's such an honour to be with everyone, and compete against Arnie (Titmus). She is an absolute gun. She races like an absolute beast," said O'Callaghan.

"And it's an honour to train alongside her and have such a great team around us."

Haughey, who won silver behind Titmus at the Tokyo Olympics, was fastest out and led through the 150m mark before the Australian pair overtook her and went head-to-head over the final stages.

But O'Callaghan had more than her teammate when it came to the sprint finish, leaving Titmus in her wake as she touched first.

The pair have jousted at the top of the sport in the past two years -- O'Callaghan is the former world record holder but Titmus set a new mark of 1:52.23 at the Australian trials in June.

Titmus won the 400m freestyle gold in Paris earlier this week but her hopes of a "golden double" of back-to-back wins were ultimately foiled, leaving her in tears.

"These are happy tears. Honestly. I don't know. It's really hard to hold your emotions in these situations. I know what it's like to be an Olympic champion," she said.

"I'm honestly happy for (Mollie) and I'm really happy to be on the podium. I had nothing to lose tonight. I'm the Olympic champion forever in the 200m and the 400m. And I gave it everything," she added.

Haughey said she was touched by the amount of Hong Kong flags she saw after her brave attempt to outdo the Australian pair.

"This means so much to me... My whole family is here and it's just amazing knowing that I have all this support," said Haughey.

"The time was not what I was looking for but seeing the crowd really means the world to me," she said.