CANBERRA - Australian prodigy Ariarne Titmus almost broke the women's 400m freestyle world record here on Monday.

Titmus, 20, won the event in 1:53.09 at the Australian Swimming Trials in Adelaide on Monday night, qualifying for the Tokyo Olympics and establishing herself as the woman to beat at the Games.

Her time was only 0.11 seconds slower than the 1:52.98 set by Italy's Federica Pellegrini in 2009, when the now-banned super suits were allowed.

It came only one night after Titmus swam the second-fastest 400m in history, setting a time only 0.44 seconds slower than American Katie Ledecky's world record.

"It's pretty crazy," she said after the race on Monday. "It's a record that is very old. No one has been close to breaking it. Last night's swim gave me the confidence."

Emma McKeon finished second in 1:54.74 after a blistering start, and also qualified for the event in Tokyo.

Six of the eight competitors set times faster than 1:56.30, putting Australia in a strong position for the 4x200m relay in Tokyo.

Later on Monday night, Mack Horton missed his final chance to qualify for an individual event at the Tokyo Games, finishing fifth in the 800m freestyle final.

The event was won comfortably by 26-year-old Jack McLoughlin in 7:42.51.

On Saturday night, Horton came in third in the 400m freestyle - his signature event - behind McLoughlin and Elijah Winnington, and on Sunday finished sixth in the 200m.

However, the athlete could still be a part of the Australian Olympic team in the 4x200m men's relay.