Vector monitoring and arbovirus isolation are used by surveillance programs to assess disease risks to public and animal health. Adult mosquitoes trapped as part of the Victorian Arbovirus Disease Control Program (VADCP) are screened for arboviruses (such as Murray Valley Encephalitis virus and Ross River virus) using standard cell culture virus isolation protocols, and conventional molecular approaches.. Recently, an unprecedented number of unknown cytopathic agents were detected in mosquitoes collected by the VADCP. Thin sectioning electron microscopy identified highly cell-associated reovirus-like particles (~60nm diameter) in the C6/36 mosquito cell line infected with a homogenate of Culex australicus mosquitoes. Using next generation sequencing (NGS) and de novo genome assembly, the virus was identified as a member of the Umatilla virus species (family Reoviridae, genus Orbivirus). Subsequent screening identified an additional 13 Umatilla virus isolates from mosquitoes belonging to the Culex pipiens subgroup. Phylogenetic analysis of the outer capsid protein 1 gene (VP3) identified three distinct genetic variants within the collection of isolates. Isolates within each of the genetic variants were not related by surveillance season, geographical area or the mosquito species from which they were isolated. The widespread circulation of multiple genetic variants is consistent with Umatilla viruses being mosquito-borne avian arboviruses. During this study, a second unrelated orbivirus was also detected. Genetically similar to Wongorr virus, this virus was detected in Aedes camptorhynchus, a principal zoonotic arbovirus vector in Australia. Neither virus has been associated with clinical disease in humans or animals. This study documents the usefulness of virus isolation protocols coupled with NGS for the detection and identification of new arboviruses circulating in Victoria, and further supports the requirement for continual surveillance of the arbovirus landscape, to better inform public and animal health arbovirus risks assessments and provide a framework to identify biosecurity risks.