The flagship boats of both countries made their first appearances on Dorney Lake, with Australia making a bigger impression by winning the first heat in 5 minutes 47.06 seconds behind a slight tailwind. That broke Germany's Olympic-best time from eight years ago in Athens by 1.46 seconds, and the Aussies seemed in no way exerted in easily beating Germany and Canada, who also qualified.
"To lay the heat out that way gives us some confidence and sets up right up for the rest of the week," said Australia great Drew Ginn, who is bidding to win a fourth gold.
Britain, which has won this event at the last three games and is also the world champion, clocked 5:50.27 to win the second heat in fairly nonchalant fashion in Windsor, west of London. But they are no longer the dominant force in the event after a rapid Australian improvement in the past year.
"You can argue they are favorites at the moment," Britain's Tom James said. "They've won the last race between us (at the World Cup regatta in Munich), so they are a good prospect for getting a gold medal.
"But when you are racing good people and with a proven record, if you beat them, that's what you want to do."
The semifinals are Thursday, and if both countries progress to Saturday's final, it will be the most eagerly anticipated race of the regatta, given their history and rivalry in the event. Both nations have made the four their priority boat for the Olympics and there have been some choice comments directed at each other in the past week, particularly from the outspoken Ginn.
"They play their game like we play ours. It's good to hear," Britain's Andrew Triggs Hodge said. "They've got a lot of fight in them, it's just what we need for an Olympic Games."
The United States won the third heat in 5:54.58 and could also be a contender for gold. The U.S. has strengthened its four for London and is something of an unknown quantity having not raced internationally this season.
The men's four will also be the final race of the eight-day meet.
Earlier, Britain's Katherine Grainger's quest for an elusive Olympic gold got off to a perfect start when she broke the Olympic best in the women's double sculls with partner Anna Watkins in the first heat.
They smashed the previous best held for 20 years by Germany by nearly five seconds, crossing the line in 6 minutes, 44.33 seconds to assure their status as overwhelming favorites.
Grainger is Britain's most famous female rower, having won silvers at the last three Olympics, but should go one better at Dorney Lake with her and Watkins unbeaten since 2010.
"This has got to be one of the best days I have ever experienced in rowing and it is only a heat," Grainger said. "This is the one we want, more than anything. From any of my Olympic experiences before, this is the important one."
New Zealand, the defending champion, came second to also qualify to Friday's final. Australia and Poland advanced from the second heat, with the Aussies also going under the previous Olympic best time by 0.20 seconds.
Reigning champion Canada qualified for the final of the men's eight despite finishing second to Britain in the repechage race. Netherlands and Australia placed third and fourth to also progress to Wednesday's blue-ribbon event.
The Canadians have struggled since winning gold in Beijing in 2008 and came last in their heat on Saturday -- 12 seconds behind Germany.
The Germans are the favorites after going unbeaten since 2009. The U.S. also qualified directly from the heats.
Reigning two-time Olympic champions Romania won the repechage in the women's pair to qualify for Wednesday's final, along with second-place Germany. The Romanian boat contains Georgeta Andrunache, one of the most decorated female rowers ever with five Olympics golds across the last three games.
China, the 2008 Olympic champion, squeezed through to the final of the women's quadruple sculls by finishing as the last of the four qualifiers in a six-boat repechage. Australia won the race, while New Zealand placed last after the oar of Fiona Bourke in the third seat snapped around 400 meters from the end. The Kiwi boat was third at that point.