Kaylee McKeown has won the women’s 100m backstroke gold medal and broken the Olympic record in an “earth-shattering” final.

The 23-year-old kept her crown as she sought to defend her Olympic backstroke title against the American world-record holder Regan Smith.

American Smith set the early pace and led at the turn, with McKeown in fourth place.

“She will have to dig in here,” said Nine commentator Mat Thompson.

“Kaylee starting to lift with 20m to go. Australia’s golden girl is trying to defend a medal.”

McKeown produced a stunning last lap to surge to victory in an Olympic record time just 0.20 seconds outside of Smith’s global benchmark.

Kaylee McKeown was coming fourth before making her move. Photo: GettyAlso on Wednesday morning (AEDT), the Australian men’s relay team won bronze in the 4 x 200m freestyle with a time of 7.01.98.

Max Giuliani, Flynn Southam, Elijah Winnington and Thomas Neill put on a strong performance, but were bested by Great Britain, which took gold in 6:59.43 and the US won silver with 7:00.78.

In the heats, Kyle Chalmers booked his place in the men’s 100m freestyle final for Thursday morning, along with gold-medal winner  Mollie O’Callaghan and Shayna Jack in the women’s 100m and Zac Stubblety-Cook in the men’s 200m breaststroke.

McKeown’s triumph on Wednesday morning (AEDT) at the La Defense Arena delivered Australia a sixth gold at the Paris Games.

McKeown won in 57.33 seconds with Smith taking silver in 57.66.

Australia’s 18-year-old Olympic debutant Iona Anderson (58.98) finished fifth.

“I knew it would come down to that last 25 metres,” McKeown said.

“It’s something that I’ve been practising for and it’s something that the Americans and myself are really good at, finishing our races strong.

“So it was going to be whoever had it in that last five, 10 metres.”

McKeown is just the second woman to win consecutive 100m backstroke gold medals in Olympic history, following American Natalie Coughlin (2004, 2008).

The Queenslander collected gold in both the 100m and 200m backstroke events at the Tokyo Olympics three years ago.

She will begin the defence of her 200m crown on Thursday with heats and semi-finals ahead of Friday’s medal race.

Podium moment: Kaylee McKeown (gold) American Regan Smith (silver) and American Katharine Berkoff (bronze). Photo: Getty

McKeown has also added the 200m individual medley to her Paris  program, with the final of that event on Saturday.

McKeown now has four gold medals in her Olympic career after also featuring in Australia’s triumphant women’s 4×100 medley team in Tokyo.

She is also likely to feature in Australia’s team in the medley final to be decided on Sunday’s last day of competition at the Paris pool.

-with AAP