Ariarne Titmus says it's surreal, slaying an Olympic giant to win a famous gold medal.

The 20-year-old Tasmanian is struggling to comprehend her 400m freestyle victory which instantly entered Australian sporting folklore.

Titmus delivered Australia's second gold medal of the Tokyo Games on Monday, with Emma McKeon (100m butterfly) and the nation's men's 4x100m freestyle relay winning bronze.

The swim team has Australia's entire medal collection - two gold, a silver and two bronze.

And Titmus's gold is destined to live largest after hunting down Katie Ledecky, handing the American megastar her first-ever defeat in an Olympic individual race.

"It's surreal. It's probably the biggest thing you can pull off in your sporting career," Titmus said.

The Australian was within a whisker - 0.23 seconds - of Ledecky's world record.

Titmus snapped at Ledecky's heels for the initial 250 metres - on the surface, a cunning plan but of genuine mid-race concern.

"Honestly, at the 200 I was a little bit worried," Titmus said.

"I knew she would be prepared so I guess I had to trust myself.

"I tried to stay as composed as I could and use my speed."

Titmus ominously dialled up her pace: with 100m to go, she was within 0.16 seconds and closing.

The Australian then overwhelmed Ledecky, a renowned strong finisher who has five Olympic golds and 15 world championships in a storied career.

"To pull it off against someone who has an amazing second half of her race, I'm really proud of that," Titmus said.

"She has set this amazing standard and if I didn't have someone like her to chase, I wouldn't be swimming that way."

Titmus' triumph followed McKeon adding another medal to her collection.

After winning gold with Australia's 4x100m freestyle relay on Sunday, McKeon's 100m butterfly bronze is the sixth medal of her Olympic career - two of each colour.

The 27-year-old, a medal prospect in five more events, could depart Tokyo as Australia's most successful Olympian.

Swim legend Ian Thorpe has that honour with nine medals while Thorpe, Alicia Coutts and Shane Gould hold the Australian record for most medals at a single Olympics, five.

The men's freestyle relayers picked up a bronze on the back of a sizzling last leg by Kyle Chalmers, who was inspired by Titmus' gold.

"You stand and watch 'Arnie' win that gold medal, it boosts your energy levels," Chalmers said after posting the fastest split of any swimmer in the race.

"I know it really helped me race well, seeing an Australian winning an Olympic gold medal and beating one of the greatest swimmers of all time.

"It's really inspiring to watch and gave me goosebumps."