Considered an outsider to win the 15.5-mile dash along the streets of the southern seaport of Nice, Orica edged pacesetter Omega Pharma-Quickstep. Orica finished in a provisional time of 26 minutes, 25 seconds, with the top four teams all within 10 seconds.
The 33-year-old Gerrans, who won the third stage in a sprint finish, takes the overall lead from Belgian rider Jan Bakelants.
The Orica riders formed a circle and hugged and slapped each other on the back when they were sure of the win.
Tour favorite Chris Froome's Sky team finished third ahead of rival Alberto Contador's Saxo-Tinkoff.
Bakelants lost the leader's jersey because his RadioShack team finished way back. He narrowly avoided a crash as he came close to hitting some railings.
Froome was 3 seconds behind Gerrans, who is not considered a Tour challenger, and two-time Tour champion Contador was 6 seconds behind Froome.
The peloton returned to mainland France after three stages in the searing heat and sinewy climbs of Corsica.
Under sunny blue skies, the teams set off at four-minute intervals and the overall team standings were reversed, meaning the first team to go was Argos-Shimano and the last team down the ramp was RadioShack.
The Argos-Shimano team, including Marcel Kittel -- the German who won the Tour's hectic first stage on Saturday -- set off first. But Omega did the early damage, setting a ferociously quick time despite their best rider Tony Martin still injured by his fall on stage 1.
Garmin-Sharp, convincing winners of the team time trial when it was last held two years ago, were again among the favorites and set off with high hopes of placing veteran David Millar in the yellow jersey.
But they finished in sixth place.
There were some banged up riders taking part, too, with Martin and Geraint Thomas saddling up despite nursing some nasty injuries after crashing in the first stage.
Martin became unconscious in the team bus after his fall and was taken to hospital for injuries including bruising of the lung, while Thomas rode with a fractured pelvis.
"He is definitely better today than yesterday," Omega's team doctor Helge Riepenhof said of Martin. "Yesterday and the day before were the worst for him. There are bruises everywhere and he lost so much skin that it hurts when he moves."
Sky was 5.63 seconds behind Omega at the halfway point as Froome drove relentlessly from the front, but they left too much to do and the exhausted Thomas dropped out close to the end.
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