The precocious 18-year-old equalled her Commonwealth 400m freestyle record on Thursday night.

But what if she had some real competition?

And what if she tapered entering the national tiles?

Titmus virtually swam by her lonesome in the final in Adelaide.

She led by five seconds after 150 metres; eventually won by more than 12 seconds - an eternity in swimming language.

Titmus finished in three minutes 59.66 seconds - the exact same time she clocked at the Pan Pacific Championships last August.

And part of her wished her great rival and world record holder Katie Ledecky was in the pool too, like she was at the Pan Pacs.

'That is pretty crazy to match it," Titmus said of her time.

"I tried to do the best I can out there by myself but it's a lot tougher.

"As much as I tried my hardest tonight, you can always think 'if you did have someone there (challenging), what would happen'."

Titmus is just the third woman to break the four-minute barrier in the 400m freestyle.

Italian Federica Pellegrini was wearing a super-suit when she became the first in 2009, followed by the freakish American Ledecky who managed the feat for first time some five years later.

Titmus acknowledged that Ledecky was all-but unbeatable in the 800m freestyle.

But the Australian is increasingly bullish about her prospects of stealing Ledecky's crowns in the 200m and 400m - the American won gold in both at the 2016 Olympics, as well the 800m.

"Definitely the 400 and the 200, I have grown a lot and learnt how to race those races a lot more over the past two years," Titmus said.

"I know exactly what I have to do now going into the race, how to put it together.

"And that is something that has really helped me get my time down, knowing that feeling in the race."

Titmus earlier at the nationals set a fresh Commonwealth record in the 200m freestyle - all while training solidly for a meet which isn't doubling as selection trials for the world championships in July.

Australia's world championship team will be decided at a selection meet in Brisbane in June.

Australian Associated Press