Paris Olympics: Australia win gold and set a Games record in the women's 4x100m freestyle relay

Australia have won the women's 4x100m freestyle relay at the Olympics to join Ariarne Titmus as gold-medal winners at the Paris Games.

The freestyle relay team of Mollie O'Callaghan, Shayna Jack, Emma McKeon and Meg Harris clinched Australia's fourth-straight Oympic title in the event.

Following Titmus' triumph in the women's 400m freestyle, the relayers clocked an Olympic record time of three minutes 28.92 seconds to win from the United States and China.

Just a half-hour before the relay final, Titmus turned swimming's race of the century into a golden personal procession to defend her Olympic 400m freestyle title.

The Tasmanian is just the second Australian female swimmer to successfully defend an Olympic crown.

The legendary Dawn Fraser, who was watching from the stands at the Paris La Defense Arena on Saturday night, won 100m freestyle gold at the 1956, 1960 and 1964 Games.

'I can't really believe that's me,' Titmus said of joining Fraser.

In her much-hyped final, Titmus blitzed her rivals, American legend Katie Ledecky and Canadian teen Summer McIntosh.

The 23-year-old led from go to whoa to claim the third Olympic gold medal of her career, after her 200m-400m freestyle double at the Tokyo Games three years ago.

'I'm buggered,' she said.

'But I feel relieved. It's a different feeling winning it again after the first time.

'I know what it takes to be Olympic champion and I know how hard it is racing in these circumstances at an Olympic Games.

'It's not really like anything else, the noise and atmosphere and pressure.'

The Tasmanian-born ace clocked 3:57.49 seconds to win from McIntosh (3:58.37) and Ledecky (4:00.86).

The trio have all held the world record in the event over the past three years.

But Titmus, owner of the current benchmark, never appeared in danger of losing despite the 17-year-old McIntosh issuing the biggest challenge.

As Ledecky slipped off the pace early, the Canadian tried to keep pace with Titmus but the final margin between them - 0.88 seconds - reflected the Australian's dominance.

Titmus' fellow Australian Jamie Perkins (4:04.96) finished last.

Titmus, the world record holder in both the 200m and 400m freestyle, will next set her sights on keeping her Olympic crown over the shorter distance.

Heats and semi-finals of that event will be contested on Sunday ahead of Monday's final.

Titmus' triumph followed fellow Australian Elijah Winnington claiming the silver medal in the men's 400m freestyle, though pre-race fancy teammate Sam Short finished fourth.

German Lukas Maertens (3:41.78) took the gold medal, touching the wall some 0.43 seconds ahead of Winnington.