Rocky Elsom helped himself to a hattrick of tries as Australia opened their World Cup campaign with a 91-3 mauling of out-matched Japan on Saturday.

The Australians led 23-3 at halftime after three first-half tries to put the plucky Japanese to the sword in the second term with 10 more tries in a powerful and at times free-flowing start to the tournament.

The Wallabies scored 13 tries, 10 conversions and two penalty goals to Japan's lone penalty goal.

Elsom proved a handful for the defence while fullback Chris Latham, back from a serious knee injury and replacement Berrick Barnes - the youngest member of the squad - scored two each as the floodgates burst opened in Australia's biggest win over Japan in four meetings.

The Wallabies had the luxury of cotton-balling their playmaking fly-half Stephen Larkham 13 minutes after halftime, giving youngster Berrick Barnes his first international cap.

Barnes scored with his first touch backing up a George Gregan break to score in the 57th minute as Stirling Mortlock raised Australia's half-century with the conversion.

It became a procession to the try-line as replacement Drew Mitchell (twice), flanker George Smith, Barnes, Latham and replacement hooker Adam Freier scoring tries as the Japanese surrendered.

Japan now have only one victory in 17 encounters at the World Cup beating hapless Zimbabwe 52-8 in 1991.

Australia's only sour note was an apparent serious leg injury to replacement Mark Gerrard, just moments after he came on for Mortlock, who kicked 20 points from two penalties and seven conversions.

The Australians playing methodically built their winning platform in the first half with Elsom scattering the defence with his charges.

Elsom picked up two tries in seven minutes, one off a Larkham inside ball in the 24th minute and the other off Latham.

The Wallabies got particular satisfaction from a push-over try in the 19th minute when lock Nathan Sharpe finished off a 25m rolling maul with the Japanese unable to withstand the Australian power.

Elsom made it a hattrick of tries a minute after the resumption, striding through two weak tackles for an easy put down for Mortlock to convert for 30-3.

The impressive Latham initiated Australia's fifth try, leaping high to take a high ball and in the next ruck Larkham sent Sharpe away linking with Gregan for winger Adam Ashley-Cooper to dash away unopposed seven minutes into the second half.

Australia have beaten Japan in all their four meetings, the last time a much closer 42-23 win at the 1987 World Cup in Sydney.

It was a vibrant atmosphere inside the Stade Gerland with brass and drum bands and the appreciative crowd enjoying the autumnal sun.

Result of the World Cup Pool B match between Australia and Japan here at the Stade Gerland on Saturday:

AUSTRALIA 91

Tries - Sharpe, Elsom (3), Latham (2), Barnes (2), Mitchell (2), Ashley-Cooper, Smith, Freier; Conversions - Mortlock (7), Giteau (3); Penalties - Mortlock (2)

JAPAN 3

Penalty - K. Ono

Halftime: 23-3

Teams

Australia: Chris Latham - Adam Ashley-Cooper (Drew Mitchell 59), Stirlling Mortlock (cap) (Mark Gerrard 67), Matt Giteau, Lote Tuqiri - Stephen Larkham (Berrick Barnes 55), George Gregan - Wycliff Palu (Stephen Hoiles 67), George Smith, Rocky Elsom - Daniel Vickerman, Nathan Sharpe (Hugh McMeniman 52) - Alastair Baxter (Guy Shepherdson 51), Stephen Moore, Matt Dunning (Adam Freier 68)

Japan: Tatsuya Kusumi - Tomoki Kitagawa, Koji Taira, Nataniela Oto, Hirotoki Onozawa - Kousei Ono, Yuki Yatomi - Hajime Kiso, Takamichi Sasaki (cap), Yasunori Watanabe (Hare Makiri 12) - Luatangi Samurai Vatuvei, Takanori Kumagae - Ryo Yamamura, Taku Inokuchi, Masashiko Yamamoto

Attendance: 40,043

AAP