Projections of future population are essential for planning in both the public and private sectors but conventional approaches provide little guidance as to the uncertainties associated with published projections. Moreover, history suggests that forecasters are generally slow in responding to new trends in key demographic processes. These limitations come into sharp relief at times of high public interest, such as occasioned by the unprecedented increase in Australia's net overseas migration over the past decade. How reliable is the recent upwards revision which raises Australia's projected population from 28 million to 35 million by mid-century? If projections are to be of more value to users, progress is required on two fronts: (i) more effective processes for setting assumptions, and (ii) greater clarity as to the uncertainty of the resulting projections. We address these challenges by combining a more systematic, theoretically guided approach to the formulation of projection assumptions using a structured questionnaire to elicit the views of experts on the future of net overseas migration to Australia, and harness the results in a new, probabilistic projection framework which explicitly identifies confidence intervals within which Australia's future population growth is likely to fall. We present probabilistic projections for Australia from 2010 to 2051 and compare the results with the 2006-based Australian Bureau of Statistics projections. We show how forecast uncertainty grows, and how it varies between demographic variables.