Relaxation of predation pressure following the removal of mammalian predators has led to the irruption of large herbivores and excessive levels of herbivory in many regions of the Earth. Conservation reserves in dryland regions aim to restore ecological processes and facilitate biodiversity conservation by functioning as refuges from grazing by livestock. However, grazing pressure in conservation reserves may be high and hamper conservation objectives because wild herbivore populations have irrupted in the surrounding landscape due to predator control as well as the absence of hunting and competition with livestock within reserves. Here, we assess the relative abundance of native herbivores (kangaroos) and introduced herbivores (rabbits) using driving transects and dung surveys, and use selective exclosures to assess wild herbivores’ impacts on vegetation and soils in four conservation reserves in semi-arid Australia. Kangaroos were the dominant herbivore in each reserve. Grazing by kangaroos and/or rabbits was linked to reduced complexity of understorey vegetation, grass cover, species richness of grasses, forbs and shrubs, the depletion of soil carbon and phosphorous, and increased soil bulk density. The marked impacts of grazing by wild herbivores, particularly kangaroos, on vegetation and soils that we report are symptomatic of overgrazing. Our study provides evidence that grazing by kangaroos may jeopardize conservation efforts across a large region of semi-arid Australia. We contend that managing the total grazing pressure exerted by wild herbivores is crucial to prevent overgrazing in dryland conservation reserves where herbivore populations are not regulated by predators.