It was just the second time in her career the Tasmanian-born swimmer had equaled her Commonwealth and Australian record of 3:59.66.
Titmus lead the entire race and touched the wall almost 12 seconds ahead of second-placed Mikayla Messer (4:11.91).
“That is pretty crazy to match it,” Titmus told media after the race.
“I tried to do the best I can out there by myself, it’s a lot tougher.
“As much as I tried my hardest tonight, you can always think if you did have someone there, what would happen.”
Earlier this week, Titmus set a new Commonwealth record in the 200m freestyle with a time of one minute 54.30 seconds, which is more than half-a-second inside the previous Commonwealth benchmark.
Australia’s 2019 World Championship team for Gwangju in July will be decided at a selection meet in Brisbane a month beforehand.
At the World Champs, she will come up against Olympic Champion and World Record holder in the 400m and 800m Katie Ledecky.
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