Australia's women lowered the 4x100m freestyle world record to capture their country's second gold on the opening night of swimming competition at the Rio Olympics.

Emma McKeon, Brittany Elmslie, Bronte Campbell and individual world record-holder Cate Campbell clocked 3min 30.65sec to hold off the United States, who claimed silver in 3:31.89 with Canada third in 3:32.89.

Australia improved on the world record of 3:30.98 set by an Australian quartet at the Commonwealth Games in Glasgow on July 24, 2014.

"Definitely my favorite race ever, in an Olympic final with my sister and two girls I have known since I was 12, 13 years old and a gold and a world record. You can't ask for more," said Bronte Campbell. "Every kid dreams of it, it's what we always hoped for."

Elder sister Cate, who set the individual 100m free world record in July, said the Aussies' status as favorites hadn't led to over-confidence.

"There was no mention of gold medals or world records or placings in the lead-up," she said. "It was safe changeovers and swim your own race and focus on yourself more than anything else."

The US pulled out their biggest weapon in freestyle phenom Katie Ledecky. Although the 100m is not her speciality, the 19-year-old treble world record-holder is a fearsome competitor and when she swam the fastest leg in the heats she secured her spot in the final.

Ledecky, who tackles the 400m free on Sunday, was delighted to kick off her Rio campaign with the outing.

"It's much more fun to have a relay as the first event," she said. "We just wanted to give the Australians a fight."

Simone Manuel, Abbey Weitzel, Dana Vollmer and Ledecky claimed silver in 3:31.89.

Canada's Sandrine Mainville, Chantal Van Landeghem, Taylor Ruck and Penny Oleksiak were third in 3:32.89.

"We were there," Ledecky said. "I'm really happy with it."

Congrats Katinka Hosszú on breaking WR in women's individual medley and winning gold! Time = 4:26.36 #HUN