At Ningaloo Reef, Western Australia, jellyfishes, including ‘Irukandji’ species (small, venomous cubozoans that cause Irukandji syndrome), affect a significant whale shark tourism industry. Jellyfish are often sighted in slicks, which present as smooth, snaking lines at the sea’s surface. Slicks form from various ocean processes and accumulate organic matter. This project uses diver-operated video transects and quantitative PCR to test the hypothesis that jellyfishes are more abundant within slicks than outside slicks. Forty slicks were sampled over 20 days in one ‘jellyfish season’, in offshore waters on low-wind days. Three 100m video transects were taken within each slick and another three transects were taken adjacent to each slick. Environmental DNA samples were also taken to detect Irukandji species DNA, using new species-specific primers. The number of jellyfishes detected per video transect, and the environmental DNA of target Irukandji species, was greater within slicks, compared to outside slicks. These results suggest that slicks may be important habitat for jellyfish, including dangerous Irukandji species, and inform risk management of a significant tourism industry.