Teenage match racing skipper Olivia Price beat the Americans in a photo finish thriller then routed the British to underline her emergence as a shock sailing Olympic gold medal contender.
The 19-year-old and her crew Lucinda Whitty, 22, and Nina Curtis, 24, finished so closely locked with the US on Monday that the official timer put the margin at 0.00 seconds.
Officials awarded Australia victory after replaying race footage.
Price and her crew then destroyed world No.2 Great Britain, beating them by 22 seconds in the day's second heat off Weymouth, southwest England.
Those victories followed wins against France and Finland - ranked third and fourth in the world - on Sunday.
"It was another tricky but fun day for us ... we went out there just trying to do our normal thing and it's paid off so far," Price said.
Her stunning series of victories means she has an early share of the regatta lead with Russia, having faced and beaten the world's top four crews.
"We're trying not to focus on the winning or the losing," Curtis added.
"We're just trying to stick to our game plan and enjoy ourselves."
Price is ranked sixth in the world and entered these Games as a medal hopeful.
But their performances so far will ensure those expectations are revised upwards.
Match racing is a pits two triple-handed Elliott keelboats against each other over a short course.