White and his Brumbies teammate Christian Lealiifano were the surprise ommissions from coach Michael Cheika's World Cup squad, but with just two specialist scrumhalves in the group Larkham said that White will be called on if either Will Genia or Nick Phipps get injured.
The tenacious scrumhalf played a vital role in the Wallabies' recent victory over the All Blacks in Sydney which saw them claim the Rugby Championship title, and Brumbies coach Larkham admitted that White was understandably disappointed that he did not make the cut for the global showpiece in England next month.
"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."
With the versatile Matt Giteau able to cover scrumhalf, Cheika opted for just two specialists in that position, which meant that White missed out.
However, Larkham said that he will get the call should any of the scrumhalf options in the squad break down.
"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."
Brumbies coach Larkham said that it was just as tough to leave playmaker Christian Lealiifano out, especially given the fact that he can play in midfield as well.
"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," he said.
