Until recently, not much notice was paid to Sydney’s Sirius building. You would not find the modular concrete housing-commission flats, designed by little-known architect Tao Gofers, in any of Sydney’s architectural histories and guidebooks. Most locals would encounter Sirius only from the elevated vantage of the nearby Sydney Harbour Bridge, where they might pause in traffic to survey the building’s cascading roof terraces and curious mauve-coloured metal vents, or glance at the “One Way! Jesus” sign propped against an upper-storey window. Sirius was considered more a curiosity than a landmark, an unusual-looking public-housing complex endowed with improbable harbour views.

That all changed in 2015, when New South Wales’ Coalition government announced its intention to sell Sirius and facilitate the building’s replacement with private luxury apartments. In 2016, the then...