The Wallabies could not afford a slip-up after going down to Wales in last week's Pool D blockbuster, and returned to winning ways with a bonus-point triumph.
Australia teenager Petaia capped his long-awaited international bow with a try as the Wallabies crossed seven times to one against a spirited Uruguay team in Oita.
The Wallabies, which made 12 changes for the match before James O'Connor was replaced by Samu Kerevi on the bench prior to kick-off, started fast, despite some desperate Uruguay defending.
Australia made the breakthrough in the sixth minute, with Kurtley Beale feeding Dane Haylett-Petty for an easy five-pointer in open space as Christian Lealiifano added the extras.
The Wallabies did not have it all their own way in the first half after two yellow cards, but they appeared to play better with a man less on the field.
After Felipe Berchesi's penalty conversion and Adam Coleman's high tackle, Petaia burst through for a memorable try in the 24th minute, Beale's quick hands allowing the teenager to cross, with Lealiifano making no mistake in splitting the uprights.
Lukhan Salakaia-Loto also found himself in the sin bin approaching the half-hour mark but Tevita Kuridrani sprinted clear to put Australia 19-3 ahead after Petaia fended off two players before teeing up his team-mate.
It could have been 19-10 at half-time but Tomas Inciarte's try was chalked off by the TMO (television match official) for offside after Uruguay had celebrated and lined up for the conversion.
Kuridrani scored his second try of the match to claim the bonus point, streaking clear after collecting the ball in the middle of the field six minutes into the second half.
Will Genia came off the bench to make it 31-3 to Australia eight minutes later, with Lealiifano adding the two points, before James Slipper barged his way over to end his wait for a Test try and Haylett-Petty claimed a double 12 minutes from time.
Uruguay's hard-fought display was rewarded with a with a try at the death, Manuel Diana capping a relentless spell of pressure and 18 phases.
Australia wraps up its group-stage campaign with a clash against Georgia in Fukuroi on 11 October before Uruguay meets Wales in Kumamoto two days later.