I'm going to use names on this as it's not a blind, he's said Kimbra wasn't the first choice and there were others, it's common knowledge.
Just want to pour water on Delta Goodrem as there's no way an artist like Wally would use a fluffy pop singer like Delta. Wouldn't happen. Personally I think it's a credible indie singer and therefore people you've never heard of like Julia Stone, Sarah Blasko, Bertie Blackman, Holly Throsby...etc
I'd actually guess Missy Higgins. Whenever he toured he sung the song with Missy and I'm sure I remember hearing on a YT video of one of his tours that she was the original choice but was in the middle of producing / releasing her own album.