Circular Structure to southwest of Brisbane Observatory

Brisbane Observatory, built between 1805 and 1809, is a small, symmetrical, single-storey, three-bay private observatory. It is possibly designed by James Gillespie Graham in a neoclassical style for Sir Thomas Makdougall Brisbane, a pioneer of navigational astronomy.

The observatory has a square plan with a prominent bowed central bay to the south, which once supported a large dome. Currently, the building is roofless and has lost most of its interior, but its walls mostly stand to full height, except for parts of the south and east walls as of 2018. An excavated channel with capstones, some of which are missing, surrounds the base of the building.

The south and east elevations are faced in finely droved ashlar, while the other elevations are built of random rubble that shows signs of previous harling. The ashlar elevations feature finely moulded cornices at the cill height and eaves. The cill on the south elevation has a circular carved imprint for an astronomical instrument, and there are remnants of a blocking course in parts. A blind ashlar window is present on the east elevation.

The windows on the north entrance elevation have dressed stone margins with a projecting keystone, and the doorway is pilastered and corniced.

As of 2018, the interior is a shell, with walls stripped back to rubble and evidence of being covered with lath and plaster. A dividing wall remains to the north, separating the observatory room from a smaller retiring room, which still has fireplace openings.

Additionally, there is a small, circular open rubble structure, partially in ruins, located 11 metres to the southwest. This structure appears to date from the 19th century and is likely functionally related to the observatory.