Citizen science initiatives that collect opportunistic photos, or recordings, of living organisms (e.g. iNaturalist) are increasingly recognized for their importance in monitoring biodiversity. These projects are focussed primarily on recording the occurrence of individual species in space and time. Each photo potentially also contains additional valuable information. Here, we explored the amount and potential value of background information captured in fish photographs as a method to characterise reef habitats. The habitat in the background of fish photographs shared on iNaturalist was analysed for 6 sites across Australia. To measure accuracy of the habitat data captured in the iNaturalist photos, the habitat composition of each site was compared to standardised photo-quadrats from the citizen science project Reef Life Survey (RLS). Across all sites, 70-85% of the fish photographs from iNaturalist contained discernible biotic habitat in the background. Habitat composition as measured from the background of opportunistic fish photographs was similar to those of standardised surveys from RLS. In the face of rapid environmental change, opportunistic photographs collected by recreational divers represent a complementary way to rapidly and cost-effectively collect habitat data at more reefs and more frequently than is generally feasible with standardised scientific surveys.