The Port Stephens Koala Phascolarctos cinereus population has been regarded as one of the strongholds for Koalas in NSW. This study applied population viability analysis to investigate the impact of fire and predation by dogs on its viability. The rapid decline of the modelled Koala population under basic assumptions throws the assumed security of such large populations into question. In all the modelled management scenarios, reducing mortality had more influence than any other factor. Reducing the severity and frequency of large catastrophic fires improved the probability of survival for the population, though the modelled population size still declined sharply. Any management action to improve Koala survival must be accompanied by a reduction in mortality from dog attacks. Fires and dogs will have an ever-greater impact on Koala populations as coastal forests become more fragmented and isolated by urban development, and their combined control will be needed to complement land use planning measures to address habitat loss and fragmentation.