A new species of Platycoelostoma Morrison (Hemiptera: Coccoidea: Margarodidae) is described from the Tasmanian endemic conifer, Diselma archeri (Cupressaceae), growing in alpine heathland. Its sister species, P. compressa (Maskell), occurs in alpine and subalpine New Zealand on Libocedrus bidwilli (Cupressaceae). Vicariance dating provides a minimum age for Platycoelostoma of about 80 Ma. Cladistic analysis of morphological data from adult females and first-instar nymphs suggests that Platycoelostoma is the sister genus of Callipappus Guérin-Méneville and also is more closely related to members of subfamilies Margarodinae and Xylococcinae than to the New Zealand and South American genera of Coelostomidiinae, to which Platycoelostoma traditionally has been linked. The analysis also indicates that most traditional tribal groupings within Margarodidae are monophyletic, but that three (Coelostomidiinae, Margarodinae and Xylococcinae) of the five subfamilies in the widely used higher classification of Morrison may be either paraphyletic or polyphyletic as currently defined. Only Monophlebinae and Steingeliinae have adequate support from morphological characters. Relationships among subfamilies and many tribes are unresolved. These results lend some support to the classification previously proposed by Koteja, in which all margarodid subfamilies and a few tribes are elevated to the rank of family. Furthermore, the analysis suggests that Marchalini and Monophlebinae (in which all immature female instars have well developed appendages) are derived with respect to other margarodids and thus development via legless intermediate female instars (‘cyst’ form) may be plesiomorphic within margarodids. This hypothesis is weakly supported and requires corroboration from independent data. Platycoelostoma is transferred to tribe Callipappini, which is redefined, but the subfamily placement of this tribe must remain uncertain until the higher classification and rank of the various margarodid taxa can be re-evaluated. A revised generic description of Platycoelostoma, based on the adult and intermediate female instars and the first instar of both species, and a key to separate the female instars of both species are provided. The first-instar nymph, all female instars and the third-instar male (the prepupa) of Platycoelostoma tasmanicum sp.n. are described and illustrated. Scanning electron micrographs depict the unusual cuticular features of the third-instar female.