I think John Farnham deserves a mention, even if he does keep popping out of retirement. I will never forget his a capella rendition of "Amazing Grace", with Linzey and Venetta Fields.
http://www.youtube.com/watch?v=6danLkCJoFI
Oh, Johnnie, that was a HUGE mullet!
Venetta also sang backing vocals on Escapade, incidentally. She has a pretty distinctive sound as a backing vocalist.