Her fourth medal at the Tokyo Aquatics Centre followed a 200-400m golden double and bronze in the 4x200 relay.

The Launceston-born 20-year-old renewed her much-hyped rivalry with American Katie Ledecky who secured her own golden double by adding the 800 to 1500m with her seventh Olympic title.

"The 800 is the one that was kind of like a bonus," Titmus said.

"This is a much tougher field for me. We're all kind of around the same time, obviously Katie's world record is insane, but I knew that we'd all be around the same pace.

"I knew I had to hurt. I had to go out and try and hold on. I'm not sure if I did that.

"I tried to reel her back in at the end but I did my best and I'm pretty happy with a PB."

The Brisbane-based former Riverside and Launceston Aquatic member was second to Ledecky at every split with the gap stretching out to 2.43 at 700m. However, a trademark sprint finish left her 1.26 behind at the finish, clocking a personal best Australian and Commonwealth record of 8:13.83. Italian Simona Quadrella was nearly six seconds behind in third.

"It's good to be finished now and I'm hoping that I don't have to keep training ... surely not?

"I'm really relieved that I was able to perform here when it mattered and I'm looking forward to a good break now."

Replicating the 200, 400 and 800 freestyle medals won by Shane Gould at Munich in 1972, Titmus was proud to share the titles with Ledecky, who became both the youngest and oldest winner of the 800m.

"Even though we have a great rivalry when we race, it's not like that when we're not racing. She's a great person and a tough competitor and I know that she would have worked so hard to be where she is and I feel really honoured that I'm the person in the battle with her at the moment and I hope that I can keep that going."

The race marked the eighth and final swim of Titmus' maiden Olympic Games in which she became Tasmania's first individual gold medallist and just the third Australian after Gould and Ian Thorpe to complete the 200m and 400m freestyle double.

Earlier on Saturday, Jake Birtwhistle anchored Australia to a ninth-placed finish as mixed relay triathlon made its Olympic debut in Tokyo.

The Launceston 26-year-old produced the fastest fourth-leg run split to haul his nation into the top-10, 1:26.27 behind the gold medalists from Great Britain with the US second and France third.

Also including Emma Jeffcoat, Matt Hauser and Ashleigh Gentle, the Aussies were unable to extend a medal return which saw them on the podium at the previous six world championships, winning in 2017 and adding the 2018 Commonwealth Games title which was also anchored by the Tasmanian.

"We all love the relay," said Birtwhistle, who had finished 16th in Monday's individual event, sustaining a broken nose in a chaotic false start.

"We've been racing it for a few years now and it always delivers - a tough, entertaining event to watch and personally for us it's obviously disappointing, but that's sport I guess.

"There's ups and there's downs and unfortunately there's usually more downs than there are ups.

"But we're all excited to be out here today and we'll regroup, get ready, move on to the next one.

"We know we're a great team and deserve to be much higher up there in the results and we're capable of that. We'll put today past us and get on with the next one."

Australia was the only nation to have qualified six individual athletes for the event which saw each competitor swim 300m, ride 6.8km and finish with a 2km run.

Birtwhistle produced the seventh-fastest fourth-leg swim of 4:08, eighth-fastest ride of 9:47 and top-ranked run of 5:25 to be third quickest overall in 20:25.

The Kookaburras will face a familiar foe in the Olympic quarter-finals on Sunday.

Nine-time Olympic medallist and two-time champion the Netherlands await Colin Batch's team at 1pm.

The Dutch eliminated Australia 4-0 at the same stage in Rio de Janeiro but were also the opponents when the Kookaburras won their sole Olympic title in 2004.

Champions in 1996 and 2000, the Dutch won only two of their five fixtures to finish fourth in Pool B won by Belgium.

The Kookaburras, featuring Tasmanians Eddie Ockenden at his fourth Olympics and Josh Beltz at his first, won four and drew one to finish top of Pool A.

In the other quarter-finals, Germany play Argentina, Belgium takes on Spain and India faces Great Britain.