Eleven Brumbies players, including debutant lock Rory Arnold, made the cut for the 40-man Wallabies squad for the Rugby Championship, but Alexander's omission was the big surprise.

The other was that of his long-time front-row partner, Waratahs veteran Benn Robinson.

In their place come the uncapped Toby Smith from the Melbourne Rebels and the Western Force's Tetera Faulkner, who has two Wallabies appearances to his name.

Scott Sio (Brumbies), Sekope Kepu (Waratahs) and James Slipper (Reds) are the other props in the squad.

There is time for Alexander and Robinson to be recalled for the World Cup.

But with only five Tests between now and the opening pool game against Fiji on September 24, opportunities are limited.

Former Wallabies and Brumbies flanker Owen Finegan said it could be a case of coach Michael Cheika wanting to know how the untested young players performed on the big stage.

"The conversation with Ben Alexander and Benn Robinson was probably that you're not out of the picture yet and we want to test these young blokes and see how they go against NZ and South Africa," Finegan said.

"That will give them an idea if they are up to it.

"If it doesn't work with the young blokes, you have the two elder statesmen who both played lots of Test and Super Rugby to fall back on."

Alexander has played 72 Tests since debuting with the Wallabies in 2008.

In May the 30-year-old signed a one-year extension with the Australian Rugby Union and the Brumbies until the end of next year.

"I'm sure the decisions have been made purely on their scrummaging ability," Finegan said.

"There's been a lot of scrutiny on the Australian scrum and especially in the pool matches against Wales and England, they're probably going to be targeting our scrum more than any other part of our game.

"They obviously believe those players are the best scrummagers."

Wallabies and Brumbies outside back Matt Toomua said Alexander's omission was "notable" and "obviously very disappointing for him".

Toomua is likely to play at inside centre for the Wallabies after filling that role in the No.12 jersey in his last 15 caps for the national team.

Out of the 11 Brumbies selected, Arnold was the bolter.

The 25-year-old, who stands 2.08m, became the tallest player in Super Rugby when he debuted in round one.

"He's had a great year," Toomua said. "Early on in the season we thought we might struggle to fill a lock position, but between him and Blake Enever they did really well.

"He's obviously a great option at the line-out and gets around the park really well."

Joining Toomua, Sio and Arnold in the Wallabies squad from the Brumbies are Stephen Moore, Scott Fardy, David Pocock, Henry Speight, Tevita Kuradrani, Christian Lealiifano, Joe Tomane and Nic White.

Speight will miss the first two games of the Rugby Championship through suspension.

Alexander declined to comment when contacted by Fairfax Media.