The post office, better known as the Stock Exchange (Exchange Building, Dunedin), was Mason's greatest achievement. It was built between 1864 and 1868. Described at the time as the finest building in the colony, it was a notable aesthetic success. A two-story stone building it was tendered at 22,960 pounds, occupied most of a city block and was another palazzo, arcaded and with a 120-foot high clock tower above the central entrance. It was symmetrical in plan and overall design and was described as "Palladian with Italian and Grecian features". There was a central hall with a highly decorated interior. Its street elevations appeared effortlessly noble. Recessed columns in the arcades, elaborate stone carving in the spandrels and the rhythmic alternation of deep arches and heavily rusticated pilasters contributed to the effect, as did the recession of the first floor from the ground floor's arcade. As a consequence the first floor was set like a temple atop a mighty, balustraded plinth to which the first floor's lofty pediments added a crowning, glorious note. The cosmopolitan assurance of the design was almost startling in the context. But the building's outstanding success seems to have contributed to its demise. Transferred to the university, for which purpose it wasn't suited, it became the premises of the Colonial Bank of New Zealand, and then the Stock Exchange suffering unfortunate modifications along the way. It was a mess of ill-considered additions when it was demolished in 1969.