Wallabies utility Mark Gerrard's World Cup is over after rupturing his medial knee ligament in his first touch of the ball, souring Australia's 91-3 thumping of Japan today.

Gerrard won't need an operation for the dreadful injury to his left knee but will be sent home to Australia in the next couple of days.

He came off the bench in the 68th minute and had just one minute on the field before being carried off Lyon's Stade Gerland.

The 25-year-old winger took the ball into the Japanese defence and finished in considerable pain.

Gerrard was rushed away for scans and coach John Connolly was fearing the worst immediately.

Adam Ashley-Cooper was another casualty with a badly bruised toe and is in doubt for Saturday's Pool B game against Wales in Cardiff.

The Wallabies finished today's bloodbath with just 14 men but it didn't stop them from scoring three tries in the last 10 minutes against the outgunned Cherry Blossoms.

Fired-up flanker Rocky Elsom quickly answered criticism from former Test coach Eddie Jones with a hat-trick of tries in the World Cup opener.

Elsom took just 41 minutes - the fastest by a forward in the tournament - to score three of Australia's 13 tries.

The backrower's menacing display equalled Toutai Kefu's 1999 effort against Romania for the most tries scored by a Wallabies forward in a World Cup match.

Elsom responded to pre-tournament jibes by former Wallabies coach Jones about his work-rate by scoring two of Australia's first three tries to take a 23-3 halftime lead in front of 40,043 fans.

Debutant Berrick Barnes, 21, also had a day to remember by scoring a double, including his first after just two minutes on the ground following his 57th minute substitution of Stephen Larkham.

Fullback Chris Latham, in his first Test run-on start this year, and replacement winger Drew Mitchell also bagged two tries each.

Mitchell was on for less than a minute when he crossed midway through the second half for Australia to jump to 58-3.

Seven tries were scored in the last 23 minutes for the Wallabies' second biggest World Cup win.

Despite the scoreline, the Cherry Blossoms showed from the outset they were up for a fight when they, to a man, aggressively joined a sixth-minute scuffle between George Gregan and lock Takanori Kumagae.

It provided the third huge roar from the pro-Japan crowd within as many minutes.

The first was when the twisting Japanese scrum somehow retained its own ball.

Then winger Tomoki Kitigawa managed to spark a rollicking 50m counter-attack after a Larkham cross kick bounced on the tryline and evaded three Australians.

But they had no answer to the bigger, stronger and more skilled Australians as the match wore on.

Japan coach John Kirwan defended his decision to select a second-string side and fired back at the IRB for scheduling his team's second match against Fiji in just four day's time.

"You need to ask the IRB that question, whether they had any regrets about giving us a draw that was unfair," Kirwan said.

AAP