The female of Delias aestiva smithersi Daniels, 2012 comb. nov. from northern Australia is illustrated and described for the first time. The subspecies is diagnosed and its taxonomic status clarified. Several unique character states concerning wing colour pattern elements of D. aestiva smithersi, together with evidence of the male genitalia, support the hypothesis that the subspecies belongs to D. aestiva Butler, 1897 and not to D. mysis (Fabricius, 1775). The taxon appears to have a restricted geographical range, being limited to the eastern Gulf of Carpentaria on the western side of Cape York Peninsula, Queensland (from Weipa to Karumba), where it occurs in mangrove habitats in coastal lowland areas. Delias aestiva smithersi and the nominate subspecies, D. aestiva aestiva Butler, 1897 from the “Top End”, Northern Territory, are allopatric and geographically separated by the Gulf of Carpentaria, suggesting that this biogeographical barrier (the Carpentarian Gap) has facilitated differentiation within the species, either though vicariance or dispersal.