White was arguably the most contentious omission from the Wallabies' 31-man squad – less than a fortnight after his match-winning cameo off the bench against the All Blacks in Sydney to secure The Rugby Championship.

But after White started in the No. 9 jersey in last weekend's Eden Park mauling, Wallabies coach Michael Cheika instead decided to go with Will Genia and Nick Phipps as his two scrumhalves.

Former Brumbies star Matt Giteau can also fill in at scrumhalf if there is an injury to Genia or Phipps.

White is on standby if there is an injury to Genia or Phipps before he links up with French club Montpellier for the start of a three-year contract. "He's very disappointed as is everyone who didn't make the squad," Larkham told ABC Radio Canberra.

"He performed incredibly well against the All Blacks in that first Test match and played the full 80 minutes in the second Test match and played quite well over there. It was a difficult decision, but it's one we've made and we're going to make the most of it now."

Larkham said the long and demanding World Cup campaign, and the fact the Wallabies were carrying only two specialist scrumhalves, meant White would still be in consideration.

The Wallabies will play four games in the pool stages before having to navigate through three games in the knockout phases to claim rugby's ultimate prize.

"Nic is on standby throughout the whole World Cup, so if one of those two halfbacks goes down, Nic will come into the squad," Larkham said.

"They [the players] are aware the campaign is a long one and anything can happen. There is still a good chance [White] will play in the World Cup."

White's Brumbies teammate Christian Lealiifano was another player who can consider himself unlucky to miss the final cut.

"There were a number of really difficult decisions we had to make this week and that was certainly the biggest," Larkham said.

"Christian's had a fantastic year, started out playing 12 for us then did a fantastic job at 10 when Matt Toomua came back. It was really difficult for Christian and I guess for myself [as Brumbies coach] not being able to get him in there."

The Wallabies' extended squad of 41 players will spend a week training in Sydney before being reduced for their two-week tour of the US, concluding with the Test match in Chicago.

Only two to three players outside of the World Cup squad of 31 players will travel to the US with the Wallabies, and it is understood White is unlikely to be one of those players.

There has been some suggestion a selection panel outside of the coaching staff should be responsible for picking the national team, but Larkham said the current system produced the best possible outcome.

"I think the coaches have a better understanding of the individual and how they will fit into the team environment," Larkham said. "Particularly the head coach, he has to have a vision of how he wants the team to play and therefore needs a big say in selection."