The main gallery building was originally designed by Melbourne architects Grainger & D'Ebro to house not only the art gallery but also the City Council offices, lecture theatre and public library. It is constructed of brick and plaster in an early French Renaissance style and was completed in 1887, with an extension built in 1916. It is three storeys high, with an attic in the steep pitched roofs, and a six-storey clock tower. The building was registered as a Category I heritage item by Heritage New Zealand on 24 November 1983, listed with registration number 92.