By winning the 100m title, McKeown became Australia's first women's backstroke gold medallist at an Olympic Games.

The nation had won individual events in every other stroke on the swim program, male and female.

But never women's backstroke, not in 97 years.

Until Tuesday, when McKeown mowed down her rivals with a stunning late surge to win the 100m final in Tokyo.

The 20-year-old Queenslander delivered Australia's third gold of the Tokyo Games, with a silver and three bronze also won at the pool.

McKeown turned for home in third and, in the last 25 metres, overpowered Canada's Kylie Masse and American Regan Smith.

"My legs we're definitely hurting a lot with 20 to go," she said.

"But I trained for that and knew I had a really good chance to be on the podium and come away with the position I have."

McKeown triumphed in 57.47 seconds, ahead of Masse (57.72) and Smith (58.05) to join David Theile, who the men's 100m backstroke in 1956 and 1960, as Australians to win Olympic titles in the stroke.

McKeown now sets her sights on another slice of Olympic history.

Only one Australian swimmer, freestyler Michael Wenden in 1968, has completed a 100m/200m double at an Olympics in any stroke.

McKeown is the favourite for 200m in Tokyo as she competes against a backdrop of family tragedy: her dad, Sholto, died in August last year.

Had Tokyo's Games not been postponed a year because of the coronavirus pandemic, he would have watched his girl swim at an Olympics.

"It's not necessarily what I've been through," McKeown said.

"Everyone has had a journey of their own.

"It's having that really tough mindset ... I don't think I'd be here today with all that's happened."

Australian veteran Emily Seebohm, at her fourth Olympics, finished fifth in 58.45.

"To see Kaylee there, I want to say, it was effin' special," Seebohm said.

Also Tuesday, 400m freestyle gold medallist Ariarne Titmus booked another date with American Katie Ledecky.

After their epic 400m battle, the pair won separate 200m freestyle semi-finals.

Titmus could become the first Australian woman since the great Shane Gould in 1972 to take the golden double in the 200m and 400m freestyles.

The Australian, a day after winning the 400m and swimming a 200m heat, kept herself in check for three laps before steaming to semi-final victory in one minute 54.82 seconds.

"I felt a little bit sluggish which was understandable," Titmus said.

"It was just about trying to execute a good race."

Reigning Olympic champion Ledecky then clocked 1:55.34.

The next chapter in the Titmus versus Ledecky story comes in Wednesday's final.

But the Australian warned against writing off the rest of a field including compatriot Madi Wilson, who snuck into the final in lane eight.

"I enjoy racing Katie, it's exciting for everyone and myself," Titmus said.

"But the 200 tomorrow is not just Katie and I. It's a very strong field ... it's anyone's race."

Australian Mitch Larkin was seventh in the men's 100m backstroke final won by Russian Evgeny Rylov in 51.98.