The Kangaroos overcame atrocious conditions as they scored six tries and conceded none at Langtree Park to climb to the top of the Group A.
But the result was overshadowed by a sickening injury to Lewis, who dislocated his shoulder after sliding at speed into an advertising sign with 10 minutes to go.
The Cronulla star was assisted off the field in agony and Kangaroos coach Tim Sheens said he would miss the rest of the tournament.
"It took a little while to get back in but they've sorted it. It's definitely the end of his tour.
He's a great tourer, a great member of the squad, the boys love him and he can obviously play so it'll be a big loss for us."
Sheens indicated Australia would look to bring in a replacement for Lewis.
In the Kangaroos' second group game, Daly Cherry-Evans impressed in place of the rested Cooper Cronk at halfback, scoring a try and setting up two others, while five-eighth Johnathan Thurston was named man of the match.
Josh Papalii scored a try on debut while Michael Jennings, Darius Boyd, Josh Morris and Lewis also had four-pointers.
As with their opening win over England, Australia were guilty of some uncharacteristic errors but some sloppy play was inevitable in the conditions.
Australia had the luxury of playing their tournament opener under the Millennium Stadium roof but they were well and truly exposed to the elements in front of around 14,137 fans at Langtree Park.
The match began amid driving rain, strong winds and single-figure temperatures and the conditions acted as a leveller between the sides early on but Australia's class ultimately shone through.
Fiji had never come within 50 points of Australia in three previous meetings but they did an admirable job and captain Petero Civoniceva led the charge as one of the best forwards on the park.
Both teams took time to adjust and it was Fiji who started stronger, opening the scoring with a sixth-minute penalty through Alipate Noilea.
Australia gradually took control of the game and Morris was denied a try by the video referee before Papalii scored in the 16th minute.
The Canberra forward was played into space by a lovely Cherry-Evans ball and barged his way over the line.
Boyd combined with Lewis to extend Australia's lead with a fairly soft try in the 21st minute and Lewis was again involved 10 minutes later as he combined with fullback Greg Inglis to set up Jennings out wide.
Australia were quick to add more points after halftime, with Morris scoring in the corner after Jarryd Hayne did brilliantly to bat back a Cherry-Evans cross-field kick.
Clive Churchill medallist Cherry-Evans scored under the posts in the 57th minute on the back of a James Tamou bust before Lewis finished a well-worked move in the 67th minute.
However the Cronulla star's night ended a short time after as he collided with Jennings while chasing down a grubber and there was immediate concern from his teammates.
Australia 34 (Papalii, Jennings, Boyd, Morris, Lewis, Cherry-Evans tries, Thurston 5 goals)
Fiji 2 (Noilea goal)
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