Australian swimming star Ariarne Titmus says her gold medal winning heroics are yet to fully sink in after de-throning Katie Ledecky to win the women's 200m freestyle on Wednesday.

Titmus edged out Ledecky in Monday's women's 100m freestyle final and backed that up with her second gold medal at the 2020 Tokyo Olympics.

The Australian won in 1:53:50, setting a new Olympic record, as she edged out Hong Kong's Siobhan Haughey by 0.42 seconds.

Canada's Penny Oleksiak claimed bronze, with five-time Olympic gold medallist Ledecky finishing down in fifth.

Titmus looked calm and composed in the pool after winning her second gold, with further races to come into the 4x200m freestyle relay and 800m freestyle.

"I don’t think it'll settle in until I get home and have a rest," Titmus told Australian TV network 7 after the race. "When you're in this situation you have to compartmentalise everything.

"I think once I stop racing, I think I'll release everything. On to the relay and 800m now. I don't want to ruin the rest of my meet by celebrating too hard. I'm proud with what I've achieved."

Haughey led Titmus at the final turn but the Australian stormed home over the last 50m, similar to Monday's win over Ledecky, although she said her "legs started to go".

"I was trying to mow Siobhan down in the third 50," Titmus said. "I had no idea where she was in the last five.

"I knew I had Penny covered but Siobhan was the person that was there. At the end my legs started to go but I'm happy to get it done."

She added: "Before we left, Sydney went into lockdown, it's really sad. I'm happy that the Olympics are here and we can hopefully bring some excitement to loungerooms.

"I'm fortunate to be here. I'm from a small town in Tassie [Tasmania]. It goes to show, if you believe you can do something, you can 100 per cent do it if you work for it."