Try your local vet. I would have suggested hanging it back up in the tree in an ice cream container with holes in the bottom so that it's mother could help it, but with the magpies that doesn't sound like a good idea.

Otherwise, you could contact WIRES. But usually the vets will have those contacts on hand and help the bird in the meantime. Our poor vet has ended up with many a native animal that needed assistance from us.