The country's 4x100 women's relay swim team, made up of Emma McKeon, Brittany Elmslie, Bronte and sister Cate Campbell, ensured Australia defended its Olympic title.
They clocked three minutes 30.65 seconds, beating Australia's previous world-best mark set at the 2014 Glasgow Commonwealth Games.
This came an hour after Mack Horton claimed a gold medal in the men's 400-metres freestyle race.
The 20-year-old is the first Australian since Ian Thorpe in 2004 to win the Olympic 400m freestyle crown.
Horton came in ahead of defending champion Sun Yang of China, with Italy's Gabriele Detti in third.
Australia won its first Rio medal earlier in the day when archers Taylor Worth, Ryan Tyack and Alec Potts, put on a thrilling display against China, securing a bronze medal.
