The 1958–59 modernist extension, to the west and south, is of four storeys, only one of which appears above William Street. The William Street frontage features a random patterned wall at the main entrance under a low-pitched copper-sheathed roof. The western wall is decorated with Lindsay Edward's large glass mosaic mural, , the design suggesting "primitive organic forms indicative of growth and development". On the river side is Leonard Shillam's aluminium sculpture approximately high. Entitled "Enlightenment", it depicts three figures reaching towards the sun, symbolising the dissemination of enlightenment to mankind. To the south the extension was designed as a series of concrete fin walls providing views of the river, private work areas and shade from the sun.