Little is known about the determinants of females' social interactions in species with fission-fusion social organization, in which individuals associate with different individuals at different times. Even less is known about social organization in marsupials. We investigated the association patterns of wild female eastern grey kangaroos in southeast Queensland. Female kangaroos had a nonrandom social structure and we present a new methodology for analysing social structure that accounts for the amount of spatial overlap shared by pairs of female kangaroos. This simulation model showed that patterns of association among our female kangaroos were not simply those predicted by their patterns of spatial overlap. Females that associated frequently with certain associates were able to graze for longer because they were less vigilant, and thus were able to benefit from having frequent associates. Female kangaroo social structure was affected by the loss of key members of the society during a period of unusually high predation. Information-theoretical modelling showed that the factors affecting grazing behaviour changed after partial population turnover, with the strength of association between nearest neighbours having a positive effect on grazing behaviour before population turnover and group size having a positive effect on grazing behaviour after population turnover. We propose that social organization within this marsupial is more structured than previously thought.