The cost of arms consists of a shield upon a crossed archiepiscopal cross (i.e. cross with two horizontal arms) and crosier (bishop's pastoral staff) and surmounted by a bishop's mitre. The mitre and crosier are standard ecclesiastical heraldic devices indicating the coat of arms is that of a diocese; the archiepiscopal cross indicates the diocese has the status of an archdiocese.

The red cross upon the shield divides it quarterly. The 1st and 4th quarters display the constellation of the Southern Cross (symbol for Australia) on a blue background, and the 2nd and 3rd quarters display the black swan (state emblem for Western Australia) on a gold background.