Two years after finally defeating her great American rival Katie Ledecky in the 400-metre freestyle at the world championships, the Launceston-born swimmer repeated the feat in the same event at her maiden Olympics.

It was Australia's third Olympic title in the event after Lorraine Crapp in 1956 and Shane Gould in 1972.

Defeating five-time Olympic champ Ledecky by 0.67 seconds, Titmus also claimed the accolade of being Tasmania's first ever individual Olympic champion.

"Oh my gosh, I can't believe it," said the Brisbane-based 20-year-old.

"I'm trying to contain my emotions - I've got the 200 tonight.

"This past year I don't know whether it's gone fast or slow. More than anything, to get here was a relief and then to come here and do the job, I'm over the moon."

Asked by Channel Seven what she had said to Ledecky after the race, Titmus said: "I thanked her. I wouldn't be here without her. She set this incredible standard and all credit to her for the swimmer she is and I've just been trying to chase her and it's really exciting now that we have this battle going. It's really fun to race and that's the best part about it. In the race I tried to stay as composed as I could, to stick to my race plan, and I can't believe I actually pulled it off."

The former Riverside and Launceston Aquatic member was roared on by students at St Patrick's College, which she left to pursue her dream in 2015, as well as her parents, Steve and Robyn, sister Mia and other family members watching from Noosa.

"I just want to thank them for everything," Titmus said. "None of this would have been possible without them. Moving to Brisbane to train. It's not just my parents, it's my sister, my boyfriend, my cousins and their partners, my entire family. There's a big crew around me and I definitely couldn't do it without them."

Titmus also heaped praise on Dean Boxall, her coach at St Peters Lutheran College.

"Dean means everything to me. Coming into this race we knew exactly what we had to do. We didn't really discuss what I wanted to do in the pool, it was more of a 'have fun' moment.

"We practised this for so long, I just knew what I had to do when I got out there."

Joined by Australian swimming great Dawn Fraser to watch the event, Titmus' parents were overcome with emotion.

"I'm just so proud of her," Robyn said. "That last 100 metres, god she can race. She dug in, she never gave up. She so deserves this, her and Dean. I'm so delighted for them." Steve added: "She's just focused, determined, gritty, gutsy ... unbelievable."

Titmus, who was presented with her gold medal by Australian Olympic Committee chairman John Coates, immediately turned her thoughts to the 200m freestyle heats later the same day, before also contesting the 4x200m and 800m freestyle events.

"It was hard to contain it because I could see Dean on the other side bawling his eyes out and you don't really see that often so that made me want to tear up," she said.

"My mum is the most competitive person I know so I think I definitely get my competitiveness from her but I knew that Dad would probably be crying because he's a bit of a softie."

Tasmania's previous five Olympic gold medalists - cyclist Michael Grenda (1984), hockey player Maree Fish (1988), rower Stephen Hawkins (1992), hockey player Matthew Wells (2004) and rower Scott Brennan (2018) all won as a member of teams.