The unsung Stubblety-Cook has won Australia's first Olympic gold in the 200m breaststroke since the 1964 Games, also in Tokyo.

Also Thursday, Australia's 4x200m freestyle relay team claimed bronze.

But Chalmers was an agonising 0.06 seconds from defending his 100m freestyle title, taking the silver medal.

"It's bitter-sweet," Chalmers said.

"To get second is amazing, to back it up after gold in Rio ... but to be so close, it does hit home a little bit."

American Caleb Dressell (47.02 seconds) touched out Chalmers (47.08) with Russia's Kliment Kolisnikov (47.44) third.

Chalmers, who had shoulder surgery last November, was seeking to become the first Australian to win the event at two Olympics.

"I have had 12 cortisone injections in my left shoulder, and the surgery ... I have had ankle problems, three epidurals in my back, cortisone in my right shoulder," he said.

"That's just the physical side of things. Everyone has challenges.

"But to stand up and go for an equal-best time in an Olympics final, when it counts the most with all the pressure and expectation on me, it is special."

Chalmers' silver followed Stubblety-Cook's gold, Australia's first in the men's 200m breaststroke since Ian O'Brien in 1964.

Stubblety-Cook triumphed in two minutes 06.38 seconds from Arno Kamminga (2:07.01) of the Netherlands and Finland's Matti Mattson (2:07.13).

Stubblety-Cook was largely unheralded worldwide until producing the fastest time of the year at last month's selection trials.

"You can only be an underdog once, right? I had that luxury," said the 22-year-old.

"It was an experienced field but through the heat and semi it was quite exciting that I had a little bit more to give.

"I was happy enough to be here as this time last year we didn't think the Games were going to happen."

In the women's relay, Australia flipped its entire team from the heat with Ariarne Titmus and Emma McKeon winning their third medals of the Games.

Titmus, McKeon, Madi Wilson and Leah Neal couldn't match China, who won gold in a world record 7:40.33.

The previous world record was 7:41.50 set by Australia at the 2019 world championships.

In the Olympic final, Titmus, the 200m and 400m freestyle champion, McKeon, a 4x100m freestyle relay winner, Wilson and Neale finished third in 7:41.29 with the United States second in 7:40.73.

Titmus now has two gold and a bronze and McKeon one gold and two bronze with the prospect of another gold on Friday in the 100m freestyle.

McKeon (52.32) was fastest into the final ahead of Hong Kong's Siobhan Haughey (52.40) with Australia's opening ceremony flagbearer Cate Campbell (52.71) next-best.

In the men's 800m freestyle final, Jack McLoughlin, the 400m freestyle silver medallist, finished last.

Australia's Tristan Hollard (200m backstroke), Jenna Strauch (200m breaststroke) and Mitch Larkin (200m individual medley) failed to advance after their respective semi-finals.

Australian Associated Press