Wheat streak mosaic virus (WSMV) was found for the first time in Australia in 2002. It subsequently was found widely dispersed around the continent and was shown to be seedborne in wheat. The coat protein (CP) gene sequences of nine WSMV isolates from eastern and southwestern Australia are reported, one obtained directly from infected wheat seed, three from seedlings grown from infected wheat seed, and five from infected wheat plant samples. These sequences were compared with those of 66 WSMV CP sequences, including eight previously sequenced Australian isolates. All 17 Australian sequences formed a closely knit monophyletic cluster as part of the D1 subclade of WSMV previously only reported from the Pacific Northwest of the United States. The close phylogenetic relationships of these sequences indicate that the Australian outbreak arose from a single incursion, the source of which appears to be the Pacific Northwest. Three Australian CP sequences were identical, one from the location of the post-entry quarantine facility at Tamworth, New South Wales, and two from seed that had originally been propagated at that facility. These three sequences were closest to the Pacific Northwest sequences and differed from them by as little as eight nucleotides (0.76%). The sequence of a third seedborne isolate originally from the same source differed from the other two seedborne isolates by two nucleotides, indicating that the immigrant WSMV population may have been variable. The other Australian sequences differed from the three identical ones by only one to four nucleotides. The phylogenetic pattern and small number of nucleotide differences between individual isolates from different geographic locations fit the scenario that the virus was introduced once in seed of wheat breeding material, multiplied where it was introduced, and then was dispersed over long distances around the Australian continent along standard distribution routes for wheat breeding lines, germ plasm, and crop seed. These conclusions provide a cautionary tale indicating the importance of effective monitoring of imported plant materials for exotic virus diseases during post-entry quarantine.