Leading their opponents from Down Under by one goal with one second remaining in normal time, the USA looked certain to win when Australia struck a shot against the crossbar.
But an illegal time-out call by the USA coach led to Australia being handed a vital lifeline from the 5m penalty line, which they duly converted to take the match to extra-time.
Two USA goals in the first of two three-minute over-time periods left the Australians with too much to do, and the USA claimed victory to guarantee themselves a fourth consecutive Olympic medal.
Maggie Steffens bagged four for the Americans, as Melissa Seidemann and Lauren Wenger netted twice.
Courtney Mathewson, Jessica Steffens and Kami Craig scored one each.
Southern found the back of the net four times for Australia, as Rowie Webster scored two. Gemma Beadsworth, Glencora Ralph and Nicola Zagame bagged one each.