Dolphins live in complex social systems with a wide variety of grouping and association patterns. Understanding the spatiotemporal variation of these associations (fission-fusion dynamics) is necessary to investigate the underlying factors and mechanisms shaping mammalian social systems in aquatic environments. We used boat-based surveys, photoidentification, focal observations, association analyses and social network techniques to quantify variation in the grouping patterns and fission-fusion dynamics of small, sympatric populations of Australian snubfin dolphins, Orcaella heinsohni, and Indo-Pacific humpback dolphins, Sousa chinensis, off the northeast coast of Queensland. Schools of snubfin dolphins were larger and more stable, irrespective of behavioural activity, than those of humpback dolphins. While associations of both species showed nonrandom patterns and structure, the social network of snubfin dolphins was characterized by numerous strong associations, whereas the strength of the humpback dolphin's social network did not differ from random. Modelling of temporal patterns of association indicated long-lasting associations were an important feature of snubfin dolphins' fission-fusion dynamics. In contrast, associations among humpback dolphins over time were best described by short-term relationships. The contrasting grouping and fission-fusion dynamics of snubfin and humpback dolphins appear to be a response to different feeding habits and prey availability. Future studies involving molecular techniques and direct quantification of food availability and predation risk will help elucidate the suite of interacting ecological, social and evolutionary factors shaping their social structures.