Volunteer-collected data have become widely used, largely because of a perception of cost efficiency, however, the quality of these data is often questioned. This paper analyses the quality of visual estimates of seagrass cover collected by trained volunteers in Moreton Bay (Australia) which has c. 25 000 ha of seagrass habitat. Seagrass was routinely monitored by trained volunteers at 51 sites (50 × 50 m area of intertidal flat) using standardized protocols. Volunteers walked three transects at low tide and assessed the percentage of substrate covered by seagrass within quadrats (50 × 50 cm in area) using photographic guides. Of 33 samples (quadrats) taken at each site, nine (27%) were photographed and later scored by coordinating scientists. The visual estimation of per cent seagrass cover by volunteers was highly correlated with that of scientists and can therefore be used as a reliable source of base-line information about seagrasses in Moreton Bay. The qualities of this successful community-based monitoring programme include expert scientific and multi-organizational involvement, simple methods and result dissemination.