Some Australian mammals are so vulnerable to predation by introduced predators that they cannot yet survive outside of predator-free islands or exclosures. Populations of these animals are confined either by water or by fences, with no means for dispersal. For species with high reproductive potential, this confinement, even within large areas, can lead to overabundance and impacts to the animals themselves, to other vulnerable species and to the resources they all rely on. We outline the case of the burrowing bettong (Bettongia l. lesueur) introduced into the Arid Recovery Reserve in South Australia. Arid Recovery is a 123 km2 predator-proof fenced reserve from which cats, foxes and rabbits are excluded from 60 km2. Bettongs have increased exponentially from 30 founding animals in 1999/2000. The bettong population now has a measurable impact on vegetation within the reserve, with likely impacts to other reintroduced species such as greater stick-nest rats (Leporillus conditor) that compete for the same resources. We are researching methods to address overabundance within an adaptive framework. Options we are testing include introduction of a native predator (western quoll Dasyurus geoffroii), one-way gates to allow dispersal, and fertility control. Preliminary data are presented. In establishing confined populations, managers of reintroduction programs have a responsibility for good stewardship of populations in their care. In planning re-introductions, it is sensible to be conscious of the potential for overabundance, and to build management triggers into monitoring programs that initiate adaptive management to address the issue before there are adverse effects that become insurmountable.