Emma McKeon was the star of the show as she pulled off a stunning final leg to help Australia snatch victory from the United States at the Melbourne Sports and Aquatic Centre.

The American team led for much of the race and were under world record pace as they set their sights on gold in front of the Melbourne crowd only for McKeon to dash their hopes.

Five-time Olympic gold medallist McKeon produced a sensational 49.96 split to overhaul Erika Brown of the US as Australia featuring Mollie O’Callaghan, Madison Wilson and Meg Harris captured gold with a world-record time of 3mins 25.4secs.

The Australians beat the previous record of 3:26.53 set by The Netherlands eight years ago.

The US had to settle for silver in 3:26.29, while Canada bagged bronze in 3:28.06.

Australia's victory was then followed by another world record in the final race of the day as Italy dominated the men’s 4x100m freestyle relay final.

Alessandro Miressi came flying out of the blocks before Paolo Conte Bonin, Leonardo Deplano and Thomas Ceccon maintained Italy’s searing pace, clinching gold in 3:02.75 - taking 0.28 off the record previously held by the US.

Lani Pallister was another top performer for Australia as she put her health troubles behind her to win women's 400m freestyle gold.

The Australian was forced to undergo a procedure on her heart last year after being diagnosed with supraventricular tachycardia - a condition that can cause an irregular heartbeat.

She also developed an eating disorder before a positive COVID-19 test forced her to pull out of the World Aquatics Championships earlier this year.

But Pallister overcome all those issues to be crowned world champion in Melbourne.

With China’s defending champion and world record holder Li Bingji pulling out due to ill health, the race was blown wide open.

Pallister and Erika Fairweather of New Zealand battled it out over the first 100m with American Erin Gemmell trying to remain on their tail.

There was nothing between Fairweather and Pallister, matching each other stroke for stroke going into the final six laps.

With the home fans getting behind her, Pallister managed to edge ahead with three laps left and held on brilliantly to secure gold for Australia, winning in 3:55.04.

Fairweather had to settle for silver in 3:56.00, with Leah Smith of the United States coming through for bronze in 3:59.78.

The women's 200m individual medley final turned out to be an all-American duel between Kate Douglass and Alex Walsh.

Olympic bronze medallist Douglass took the early lead before Walsh came back fighting with a brilliant backstroke.

The pair slugged it out with Douglass managing to keep her nose in front over the breaststroke and freestyle legs before touching first in 2:02.12.

Walsh claimed silver in 2:03.37, with Kaylee McKeown ensured another Australian medal with bronze in 2:03.57.

The US hoped to continue their success in the next race as Shaine Casas got off to a storming start from lane two in the final of the men’s 200m individual medley.

South Africa’s Matthew Sates came charging back in the second half of the race, producing a superb final freestyle leg to take victory in an impressive time of 1:50.15.

Cases faded towards the end, missing out on a medal as team-mate Carson Foster dug deep to snatch silver in 1:50.96, while Canadian Finlay Knox bagged bronze in 1:51.04.

Italy’s Gregorio Paltrinieri was the clear favourite for the men’s 1500m freestyle title and did not take long to stamp his authority on the final.

Hitting the front from the off, Paltrinieri made the most of the absence of Germany’s defending champion and world record holder Florian Wellbrock to take the lead.

Japan’s Shogo Takeda managed to stay with Paltrinieri over the first quarter of the 60-lap race before fading.

Paltrinieri’s next challenger came in the shape of Norway’s Henrik Christiansen who put the Italian under real pressure.

Christiansen did not have enough in the tank though as Paltrinieri, the Rio 2016 Olympic champion, proved too strong in the closing 10 laps.

Paltrinieri powered to victory in 14:16.88 to capture his first world title and first since 2014.

France’s Damien Joly produced a late surge to take second spot in 14:19.62, while Chrstiansen claimed bronze in 14:24.08.