Titmus hit the wall at the Australian championships on the Gold Coast in 4:00.49s, in contrast to the new 3:56.08s world mark set by the 16-year-old in Toronto.

Titmus played down expectations before the race, revealing she was unwell leading into the event.

“I’ve had a bit of sickness, which has been tough trying to get over that,” the 200m and 400m freestyle Olympic champion told reporters. “I’m just battling through.”

She added after the race that her main focus was Australia’s world championship trials in June, ahead of the main event in Japan a month later.

“We want to race fast at trials and at worlds and here is more about a stepping stone and having good race practice,” she said.

McIntosh’s stunning swim broke the 3:56.40s Titmus clocked at the Australian championships last year, where she bettered American Katie Ledecky’s six-year-old world mark of 3:56.46s set at the 2016 Rio de Janeiro Olympics.

The emergence of McIntosh as a serious contender sets the scene for a blockbuster 400m clash with Titmus and Ledecky at the worlds in Fukuoka.

In other events, the versatile Kaylee McKeown stunned a strong field to win the women’s 200 breaststroke title in 2:24.18s – a stroke she doesn’t usually swim outside of the medley.

The Olympic 100m and 200m backstroke champion, whose retired sister Taylor was a breaststroke specialist, touched well clear of the more accomplished Abbey Harkin and Jenna Strauch.

“One McKeown is out so guess I have to fill in for Taylor,” said McKeown, who also swam the 100m freestyle this week, finishing eighth.

“This is a really fun event for me and think having no nerves I just gave it a red-hot crack.”

Mollie O’Callaghan, the 100m freestyle world champion, powered home in the 50m backstroke in 27.42s. McKeown holds the year’s fastest 50m back time of 27.31s but did not enter.

Cameron McEvoy won the men’s 50m freestyle in 22.11s while Thomas Neill swam sub-two minutes to take out the 200m medley and Matt Temple touched first in the 100m butterfly in 51.49s.