Microplastics are ubiquitous pollutants in aquatic environments globally. Wastewater treatment plants are considered to be a major source of microplastics and jellyfish have been proposed as potential bioindicators of microplastic pollution. We tested whether treated wastewater influenced the concentration and/or composition of microplastics in the receiving water by comparing the concentration and composition of microplastics in seawater collected in the wastewater plume and at sites distant from treated wastewater releases in the Gold Coast Broadwater, Australia, and at sites within the nearby Tweed River estuaries, which receives >10 times less wastewater discharge. In addition, tiger sea nettle Chrysaora cf. pentostoma medusae were collected to determine whether more microplastics occurred in the guts of the medusae nearby diffusers and whether the microplastics ingested by medusae were representative of those present in the water column. The concentration and composition of microplastics at the wastewater release sites did not significantly differ from sites that were distant from them. Eighty three percent of medusae contained microplastics in their guts and the composition of the ingested microplastics differed significantly from that in the surrounding water. We concluded that discharged treated wastewater had no detectable effect on levels or composition of microplastics in the receiving water and that C. pentostoma are unsuitable bioindicators because the microplastics they ingested did not represent those available in their environment.