Defining the ecological impacts conferred by invasive fishes provides a framework for evaluating the feasibility of control efforts in invaded waterways, and for predicting the consequences of future incursions. Eastern gambusia (Gambusia holbrooki) is a remarkably successful invader of freshwater systems worldwide, with the capacity to detrimentally impact native fishes both directly (e.g. competition, predation, agonistic interactions) and indirectly (e.g. triggering trophic cascades). Here, we modelled the influence of eastern gambusia and several environmental covariates on fish species diversity, abundance and condition based on quantitative survey data collected from 93 wetlands in south-eastern Australia. We predicted that small-bodied, wetland specialist species sharing dietary- and habitat-niches with eastern gambusia would be most severely impacted, and that environmental stressors associated with wetland drying during late summer would magnify these impacts. Eastern gambusia influenced the occurrence, abundance and/or body condition of most common wetland species; however, the direction and level of impact appeared dependent on both biotic and environmental forces. From these results, we postulate that generalist life-history strategies that permit niche-segregation may release some native species from competitive/predatory pressures, allowing coexistence with eastern gambusia in resource-limited, environmentally harsh habitats, whilst specialist species that occupy narrower ecological niches may be less resistant.