Emma McKeon, who swam the third leg, gave the Australian team a commanding lead over their rivals. Cate Campbell touched the wall for the Australian team in a time of 3:29.69, shaving 0.36 seconds of the previous record.
They win the first relay title in the pool ahead of Canada, who claimed silver in 3:32.78, with United States winning bronze in 3:32.81.
The Tokyo 2020 hosts are off the mark in the Olympic swimming competition, as Yui Ohashi touched home in 4:32.08 to win 400m Individual Medley gold.
Ohashi put in a dominant display in the water, leading from the early stages of the race to fend off the challenges of Emma Weyant and Hali Flickinger, both of the USA.
Tunisia's Ahmed Hafnaoui has shocked the swimming world with a stunning victory in the men's 400m freestyle final.
The 18-year-old, who just three years ago finished eighth in the same event at the 2018 Youth Olympic Games in Buenos Aires, beat off the challenge of Australia's Jack McLoughlin and the USA's Kieran Smith to win gold.
After a lightning-fast first 200m that was under world-record pace, times slowed in the second half of the race. However, Hafnaoui grew stronger over the last 50 metres, touching the wall in 3:43.36.
