Titmus hit the wall at Australia's Olympic trials in Brisbane ahead of O'Callaghan, who also went under her previous world record of 1:52.85, finishing in 1:52.48.

Titmus went out hard and held on as O'Callaghan pushed her to the limit.

"Looking at the results, it's unbelievable. I'm really happy to finally produce a swim in the 200 that I feel like my training reflects," said Titmus, 23.

"I think just the field that we have is why we are swimming so fast.

"We push each other every day. To have five girls under (trainer) Dean (Boxall) in that final is unbelievable. It's a credit to him and the programme."

Titmus stunned American great Katie Ledecky to clinch gold in Tokyo three years ago.

She will head to Paris as a red-hot favourite alongside O'Callaghan, who smashed Italian great Federica Pellegrini's 14-year-old world record last year.

The pair share eight of the 10 fastest times in history, with Ledecky, Canadian phenomenon Summer McIntosh and Hong Kong's Siobhan Haughey all in the chasing pack.

So far, McIntosh is the only other swimmer to go under 1:54 this year.

Lani Pallister and Brianna Throssell came third and fourth in Brisbane to position themselves to be part of an ominously fast 4x200m relay team.

Titmus has been in top form in Brisbane, swimming the second-fastest 400m freestyle of all time on Monday as she also prepares to defend that Olympic title.

She is also the world record-holder in that event.