Olympic champion Ariarne Titmus and the Australian women's 4x100 metres freestyle relay team have set word records in a blistering start to the swimming program in Fukuoka.

Titmus's victory in the women's 400m freestyle final at the World Aquatics Championships came only 30 minutes after Australian teammate Sam Short won the men's event.

Australia finished the evening with four gold medals after triumphing in the women's and men's 4x100m freestyle relays, giving the swimming superpower its most successful night in world championships history.

Mollie O'Callaghan, Shayna Jack, Meg Harris and Emma McKeon stopped the clock in three minutes and 27.96 seconds, handing Australia its third straight gold in the event.

The time broke the previous world record (3:29.69) held by Australia, set at the Tokyo Olympics in 2021.

Only minutes later, the men's relay quartet — featuring Jack Cartwright, Flynn Southam, Kai Taylor and Kyle Chalmers — struck gold in 3:10.16.

Australia's night was soured, however, with Kaylee McKeown — who won both individual backstroke events at the Tokyo Olympics — disqualified in the women's 200m individual medley semifinals for an illegal turn between the backstroke and breaststroke legs.

Having lost her world record to Canadian teenager Summer McIntosh earlier this year, Titmus produced a breathtaking performance to win the 400m freestyle title in a time of 3:55.38.

Four-time world champion in the event Katie Ledecky of the United States took silver, more than three seconds adrift of Titmus in 3:58.73.

New Zealand's Erika Fairweather (3:59.59) was third, while 16-year-old McIntosh — who had held the world record with a time of 3:56.08 — was fourth in 3:59.94.

"I was just really excited for this race," Titmus told the Nine Network.

"I've put in a lot of work the last six weeks and I took a lot away from [the national] trials. I swam the way I wanted to.

It is Titmus's second world championships gold in the 400m freestyle, having first won the event in 2019 in Gwangju.

Earlier, Short was locked in a enthralling battle with Tunisia's Olympic champion Ahmed Hafnaoui in the final lap of the men's 400m freestyle final, before touching the wall in first place.

The 19-year-old Queenslander swam 3:40.68, beating Hafnaoui by just 0.02.

"Bloody amazing," Short said after the final.

"I knew it was going to be a race in two with 100 (metres) left. I got goosebumps thinking I was in the race with him (Hafnaoui) and I got my hand on the wall first.

"It hasn't sunk in yet."

Short is the fifth Australian to win the event at the world championships.

German Lukas Märtens (3:42.20) claimed bronze, while Australia's defending champion Elijah Winnington (3:44.26) was seventh. 

Among other events, France's Léon Marchand broke Michael Phelps's long-standing world record in the men's 400m individual medley in winning gold.

Marchand swam 4:02.50, eclipsing Phelps's mark of 4:03.84 that the American great set at the 2008 Beijing Olympics.