I hate to say it's your fault, but class of travel is indicated very clearly on purchase so what you have most likely done is purchase a mixed cabin ticket without looking carefully.

If you were downgraded via email it would be a different story but that's not really what happened to you is it?

Also did you try show your long haul BP at the lounge in Melbourne? Most carriers allow it on mixed cabin flights if you just flew a long haul and are connecting short haul - although without knowing your airline one can only guess.