The ecology, life history and morphometrics of the Australian trombiculid mite Guntheria coorongensis (Hirst, 1929) have been studied. Larvae were captured ectoparasitic on Rattus fuscipes greyii in the south-east of South Australia and successfully reared to protonymphs and deutonymphs, which are described. Comparisons are made of the larvae with the type series from South Australia, as well as with other Australian larvae and deutonymphs previously synonymized with this species, using morphometric criteria. Significant differences are found between the metric data of both larva and deutonymph from Guntheria cairnsensis (Womersley and Heaslip, 1952), so that the latter species is restored nomenclaturally. Teratological abnormalities of the reared deutonymphs of Guntheria coorongensis are recorded.