Flabbergasted former Wallabies captain and selector Andrew Slack can't fathom why Matt Giteau is set to be a controversial omission from Australia's World Cup squad.

Giteau was the Wallabies' highest-paid player and star playmaker until last year but coach Robbie Deans will controversially overlook the 28-year-old when he announces his 30-man Cup squad on Thursday.

This is despite the 92-Test stalwart being able to cover three positions, including halfback where Deans is expected to include rookie Nick Phipps as a third specialist No.9 behind general Will Genia and back-up Luke Burgess.

The widespread understanding that NSW playmaker Berrick Barnes would pip Giteau for the last backline place in the squad was confirmed on Wednesday afternoon when the France-bound back posted a leading comment on his Twitter account.

"Thank u ballboys, thank u linesman! That's it", Giteau tweeted.

He later confirmed his non-selection with another tweet.

"Really wanna thank everyone for the support! Was a huge honour to get the chance to represent my country as often as I did. veryprivileged"

It means Deans has shown full faith in Barnes standing up as the only specialist playmaker to back up incumbent Quade Cooper, just four club matches back from the concussion problems which have clouded his career.

He is highly regarded by Deans, who appointed him vice-captain ahead of the 2009 spring tour before Barnes withdrew through injury.

Giteau was the man overlooked for the role at the time and his unhappiness showed.

That reaction, as well as a failure to close out several close losses as playmaker and goalkicker in 2009-10, seem to have cost him dearly.

But Slack, who led Australia to their historic 1984 Grand Slam success in the United Kingdom, didn't see Giteau as out of form or a destabilising influence, and deserved selection.

If there had been a genuine fall-out between Deans and Giteau, the 39-Test centre said the pair needed to man up and mend their bridges.

"From outside the team environment, I can't see any reason why he wouldn't be picked," Slack told AAP.

"I know Giteau was close to being the Australian captain for the 2007 World Cup but it's just got me flabbergasted.

"They're only scuttlebutt/rumours but, if they don't get on, one of them needs to grow up, or both of them. If that is the issue, then that's a real shame because at the end of the day, the only people who end up paying for that is everyone else.

"Talent, experience, class, versatility and healthy (match fit) - they're five compelling factors I would have thought."

Barnes is among six players with question marks over their fitness, and Slack felt Giteau was a much safer bet as a back-up to X-factor Cooper.

"The other thing I think is the big seller is this third halfback; when you pick a specialist third halfback (like Phipps) when you've got a Giteau ... is just crazy," he said.

Giteau's close friend Drew Mitchell - who'd suffered an ankle injury during the Super Rugby season - indicated through Twitter that he'd been selected but the winger was feeling sorry for his mate.

"The song of the day is Bittersweet Symphony by The Verve," he tweeted.

Hooker Tatafu Polota-Nau (knee), prop James Slipper (ankle) and centre Rob Horne (elbow) are all expected to join Mitchell in the squad after undergoing fitness testing on Wednesday but No.8 Wycliff Palu (shoulder) is in doubt.

Reactivated Radike Samo may sneak in ahead of Palu while veteran lock Dan Vickerman is also crossing his fingers.

Possible World Cup squad: Backs - Kurtley Beale, James O'Connor, Drew Mitchell, Digby Ioane, Adam Ashley-Cooper, Rob Horne, Pat McCabe, Anthony Faingaa, Berrick Barnes, Quade Cooper, Nick Phipps, Luke Burgess, Will Genia.

Forwards - Ben McCalman, Radike Samo, David Pocock, Matt Hodgson, Rocky Elsom (capt), Scott Higginbotham, James Horwill, Rob Simmons, Nathan Sharpe, Dan Vickerman, Ben Alexander, Sekope Kepu, James Slipper, Ben Daley, Stephen Moore, Saia Faingaa, Tatafu Polota-Nau.

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