This paper considers the relationship between the cultural histories of Canberra, selected as the site for the Australian capital in 1908, and Milton Keynes, the British new town designated in 1967. Both are master-planned cities designated on greenfield sites, incorporating formal experimentations in low-density urban design; they have also both been reviled as inauthentic and undesirable, and are often likened to each other as examples of urban planning failure. Despite these cultural associations, the entwined receptions of Canberra and Milton Keynes have not yet been traced historically. Reading across their media and cultural representations during the late twentieth century, this paper proposes that these cities have symbolically functioned to displace and contain broader cultural anxieties about imperial and post-imperial urban authenticity. The persistent critiques of newness levelled at Canberra and Milton Keynes help maintain deeply engrained mythologies of urban value deriving from norms of “immemorialist” heritage and historical continuity, normalising and conferring legitimacy on the interventionist mechanisms of theft and destruction which built British imperial cities. The paper concludes by reflecting on the political function of containing anxieties about authenticity to maintain and reaffirm fictions of imperial ignorance.