The I'd Do Anything winner is leading the search for the dog to play the famous role in Webber's West End adaptation of The Wizard Of Oz.
Two open auditions for dogs will be held in April, where ten canines will be selected to meet with Webber.
"As a cat man, it's been one of the most difficult things I've ever had to do," he joked. "All these Totos have been thrust in my direction."
Graham Norton, who will host the main show's search for a Dorothy, said: "The only thing everyone wants to know about is the search for the dog. Well, lovely Jodie Prenger will be heading up the search for a Toto with a panel of dog experts.
"The auditions will whittle them down to ten dogs and they will be invited to meet Andrew Lloyd Webber. Of the dogs that accept that invitation - some could be busy, some can't forgive him for Cats - Andrew helped by the dog experts will choose five dogs. And we'll then follow those five through the series."
He joked: "Don't worry - all our Dorothy's have signed wavers in case they get rabies!"
Over The Rainbow starts with a weekend special on March 26 and 27 on BBC One.