Benthic cyanobacteria blooms, including those of the nitrogen-fixing species Lyngbya majuscula, appear to be becoming more numerous and widespread in marine habitats worldwide, and have negative impacts on the environment and human health. The progression of a L. majuscula bloom in south-east Queensland, Australia was mapped along with intensive biomass and tissue nutrient sampling every 10–14 days over the bloom’s 3.5-month duration in summer 2005–2006. Data-integrated GIS maps illustrated the changes in biomass and tissue nutrient pool of the L. majuscula through different growth phases (incipient, rapid expansion, plateau or peak and decline) of the bloom. At the peak, L. majuscula covered 509 ha and had a mean density of 115 g_dw m^–2, with the maximum density recorded 503 g_dw m^–2. The highest mean total carbon (29.4% C), nitrogen (3.5% N) and phosphorus (0.143% P) contents in L. majuscula tissue corresponded with the peak in biomass. Three-dimensional modelling calculated that at the peak, the bloom contained 5057 t_ww (510 t_dw) of L. majuscula; 150 000 kg C; 18 000 kg N; 720 kg P; and 5200 kg Fe. This information gives an insight into L. majuscula bloom dynamics and ecophysiology and provides quantitative data for models.