I'm not familiar with Victorian property law, but from your use of the word 'setback' it occurs to me that you might not be referring to a covenant so much as to Council's building and development requirements. In granting building approvals Councils will have minimum distances from boundaries in which they either won't allow construction, will only allow certain types of construction, or will allow it but perhaps with certain conditions. These minimum distances are what are commonly referred to as setbacks.
They're not registered on title, and not part of any contract of purchase. Rather, they are pursuant to the Council's powers under their version of the local government legislation.