Team includes sisters Bronte & Cate Campbell; the Aussies had also won in London four years ago
Australia's women, powered by sisters Bronte and Cate Campbell, beat the United States and Canada to win the Olympic 4x100 metres freestyle relay in world record time on Saturday, retaining the title they won in London four years ago.
The Australians, leading off with Emma McKeon and Brittany Elmslie swimming the second leg, clocked three minutes, 30.65 seconds, beating the previous mark of 3:30.98 they set in Glasgow in 2014.
The medal came on top of a win for Mack Horton in the men's 400m freestyle.
It meant that after one day of competition in the Rio pool, Australia have already doubled their tally of golds from a disappointing Games in London in 2012.
The Australian women won by an emphatic 1.24 seconds from the American team of Simone Manuel, Abbey Weitzeil, Dana Vollmer and Katie Ledecky.
"I think the swim team is in a very good place - doubled medal tally from London," Horton told a news conference. "The youth of the swim team brings a lot of potential, I think that's why we're in such a good place."
The Americans had led at half-way, but Bronte Campbell overhauled Vollmer on the third leg and handed over a lead that sister Cate never looked like surrendering to Ledecky.
Canada's Sandrine Mainville, Chantal Van Landeghem, Taylor Ruck and Penelope Oleksiak took the bronze ahead of London silver medallists the Netherlands, who were anchored by triple Olympic champion Ranomi Kromowidjojo.
The Dutch team's third leg was swum by Inge Dekker, taking part in her fourth Olympics, who was diagnosed with cervical cancer in February and underwent surgery in March.