Sydney - Coach Michael Cheika on Friday said the Wallabies planned to be "unpredictable" in Japan as he tapped uncapped back Jordan Petaia, 19, to become Australia's youngest ever Rugby World Cup player and picked Adam Ashley-Cooper for his fourth campaign.

Cheika opted for just two halfbacks, Will Genia and Nic White, in his 31-man World Cup squad, meaning Brumbies No 9 Joe Powell and the Waratahs' Nick Phipps miss out.

Another notable absentee was veteran hooker Tatafu Polota-Nau, but flank David Pocock, who hasn't played for seven months after injuring his calf, was named, along with Bernard Foley and Tevita Kuridrani who were overlooked in recent games.

Australia struggled through one of their worst years in decades in 2018 and have lost the services of star fullback Israel Folau, who was sacked in May for posting homophobic comments online.

But this month's record 47-26 drubbing of New Zealand hinted at a revival, despite last week's chastening 36-0 defeat in the return Bledisloe Cup fixture in Auckland.

"We've become a really tight group and it was a very difficult process in picking only 31 players, but I know each player selected will travel to Japan with the full support of those team-mates who won't board the flight," said Cheika.

"We have worked really hard at building each week and each game so far this season, and play a brand of rugby that Australia would be proud of and get every player contributing towards the outcome."

The line-up, captained by Michael Hooper, boasts 1 406 international caps, 200 more than the squad which reached the final in 2015.

Eighteen players will be tasting World Cup action for the first time, in contrast to the 35-year-old Ashley-Cooper, who will become only the second Wallaby after George Gregan to play in four.

The dynamic Petaia can play from the wing or outside centre, but his inclusion carries some risk after he missed much of the Super Rugby season with injury.

The Queenslander is expected to make his debut in Australia's final warm-up Test against Samoa in Sydney on September 7, where Pocock is also tipped to return.

"I'd say we'll be a little bit unpredictable in Japan, that's a bit of our theme this year," said Cheika.

"At the World Cup, every game is a grand final. And how you approach that - you don't look any further down the road that what's in front of you," he added.

The Wallabies, beaten in the 2015 final by New Zealand but now ranked just sixth in the world, have had mixed fortunes so far this year, losing to South Africa in the Rugby Championship before beating Argentina.

Despite last week's crushing defeat to New Zealand their form has been an improvement on 2018, when they played 13 Tests and won just four.

In Japan, they face a tough Pool D that includes world number one Wales, Georgia, Fiji and Uruguay. They open their campaign against Fiji in Sapporo on September 21 ahead of a crunch clash with Wales a week later.

Asked what his message was to the team in Japan, skipper Hooper said: "Make it count."

"You never know when you might get another opportunity like this," he added.

"Make the most of it, love every day and enjoy it because if you're enjoying it, it usually makes for good rugby too."

Australia's 2019 Rugby World Cup squad:

Forwards (17)

Allan Alaalatoa, Rory Arnold, Adam Coleman, Jack Dempsey, Folau Fainga'a, Michael Hooper (captain), Sekope Kepu, Tolu Latu, Isi Naisarani, David Pocock, Izack Rodda, Lukhan Salakaia-Loto, Rob Simmons, Scott Sio, James Slipper, Taniela Tupou, Jordan Uelese

Backs (14)

Adam Ashley-Cooper, Kurtley Beale, Bernard Foley, Will Genia, Dane Haylett-Petty, Reece Hodge, Samu Kerevi, Marika Koroibete, Tevita Kuridrani, Christian Lealiifano, James O'Connor, Jordan Petaia, Matt Toomua, Nic White