Titmus has collected her second gold and a rare Olympic double, winning Wednesday's 200m freestyle final after her 400m freestyle triumph.

Only two Australians - swim legends Ian Thorpe (2004) and Shane Gould (1972) - have completed the 200m-400m freestyle double at an Olympics.

The 20-year-old also joined the great Susie O'Neill as only the second Australian woman to capture 200m freestyle gold on another glorious day for the Dolphins.

Australia's men's 4x200m freestyle relay team also picked up a bronze, taking the swim team's haul to four gold, one silver and four bronze.

And Titmus has the 800m freestyle and 4x200m freestyle relay to come.

The 20-year-old could join Australian swimming's elite half-dozen to win three golds at a single Games.

"I'm not going to let it settle down too much, I'm only halfway through my program, still got the relay and 800 to go," Titmus said.

"I've got the afternoon off now. Have a rest, come back in for a light swim and get ready for tomorrow."

Titmus' 200m final was billed as the next chapter in her rivalry with Katie Ledecky, a five-time Olympic champion from the United States.

The pair's dual over 400m instantly entered Olympic folklore but Titmus' challenge in the shorter distance on Wednesday came from elsewhere.

Ledecky finished off the pace in fifth as Siobhan Haughey from Hong Kong led until the last 10 metres, when Titmus's trademark finishing flurry reeled her in.

Titmus timed her surge to perfection, winning in one minute 53.50 seconds ahead of Haughey (1:53.92) and Canadian Penny Oleksiak (1:54.70).

"Having a great swim in the 400 gives me confidence coming into this 200," Titmus said.

"The back end was definitely my strength in the 400 so I knew I would have that on the way home in the 200.

"But every race is different. Can't rely on that confidence too much.

"I have still got to execute a great race."

In the men's relay, Alexander Graham, Kyle Chalmers, Zac Incerti and Thomas Neill took bronze behind Great Britain and the Russian team.

Australia was fourth when Neill entered for the last leg and the 19-year-old produced the nation's fastest split of the final.

Great Britain won in six minutes 58.58 seconds followed by the Russians (7:01.81) just ahead of Australia (7:01.84).

Earlier, Chalmers, the only Australian individual trying to defend an Olympic title in Tokyo, advanced into the 100m freestyle final.

The South Australian was sixth-fastest into Thursday's final, with Russian Kliment Kolesnikov taking top billing through the semi-finals in 47.11 seconds - some 0.37 inside Chalmers' winning time at the 2016 Rio Olympics.

American star Caleb Dressel (47.23) and Italian Allesandro Miressi (47.52) were next best with Chalmers finishing in 47.80.

In the men's 200m butterfly, Zac Stubblety-Cook enhanced his gold medal claims with an emphatic semi-final victory.

Stubblety-Cook, who entered Tokyo with the world-best time in the event this year, led the way into the final in 2:07.35, with Great Britain's James Wilby next-best in 2:07.91.

And Australia's Brianna Throssel booked a berth in the 200m butterfly final, logging the sixth-fastest time through Wednesday's semi-finals.

Australian Associated Press