Seeds of three important cucurbit species (cucumber, melon and zucchini) were obtained through on-line mail-order providers and tested for selected viruses of biosecurity and agronomic concern. One or more of the targeted viruses were detected in 23 of the 31 seed lots tested. The quarantine pest melon necrotic spot virus was detected by ELISA in melon seeds. Multiple instances of contamination with the quarantine pest cucumber green mottle mosaic virus were also detected, and its presence in seeds of all three plant species was confirmed by inoculation and re-isolation from indicator plants. Cucumber mosaic virus, squash mosaic virus and potyviruses were repeatedly detected, and instances of insect pest and plant trash contamination were also observed. Seeds imported into Australia from some on-line sources have potential to introduce organisms of biosecurity concern.