Horse flies (Diptera: Tabanidae) are recognised for their medical and veterinary importance, but they also have an important role in pollination. The genus Scaptia Walker 1850 contains more than 100 species and comprises seven subgenera with an exclusively southern distribution from Australia, New Zealand, New Guinea and South America. Five new Australian species are described in the subgenus Plinthina, which previously comprised only seven species, and an existing key is modified to include the new species, all of which are diagnosed and figured. The new species are: S. (Plinthina) arnhemensissp.n. Lessard, S. (Plinthina) aurifulgasp.n. Lessard, S. (Plinthina) beyonceaesp.n. Lessard, S. (Plinthina) nelsonaesp.n. Lessard and S. (Plinthina) nigripunctasp.n. Lessard.