In 2002, Tasmania reported the largest number of Ross River virus (RRV) infections ever recorded for the state. Of the 117 cases, 37 lived in, or had visited, the Sorell Municipal Area, east of Hobart. In early 2002, a combination of spring tides and high summer rainfall produced extensive salt marsh habitat in the Sorell region, resulting in unseasonably high densities of the mosquito Ochlerotatus camptorhynchus, recognised vector of RRV. Four isolates of RRV were identified from collections of adult mosquitoes. All four isolates were from Oc. camptorhynchus, collected near the Carlton River. This is the furthest south that RRV has been identified in Australia and the first identification from south-east Tasmania. The virus carriage rate in the mosquito vector populations were very high, with successive weekly minimum infection rates of 17.1, 3.0 and 11.1 per 1,000 Oc. camptorhynchus at Carlton River from mid-February to early March. The first isolation of RRV from mosquitoes coincided with the onset dates of the first human cases of RRV infection.