As we move through the demographic transition, an older age structure is inevitable but future possible scenarios vary. Official Australian Bureau of Statistics (ABS) population projections for Australia show that, depending on varying assumptions for fertility and migration, the population in 2051 could be large and growing or the same size as it is today and declining. But an analysis of these same projections confirms earlier work which finds that migration has a minimal effect on the age structure. The paper analyses nine different population projections produced by the ABS, four in 1996 and five in 1998. The highest projection (fertility 1.85 and net migration 100,000) adds an extra ten million people by the year 2051 while the lowest (fertility 1.6 and nil net migration) adds none. This is a dramatic difference. But the effects of different fertility and migration assumptions on the median age in 2051 are also marked; changes in fertility have a strong effect while changes in migration have a weak effect. The projection which produces the lowest median age in 2051 for the least ‘cost’ in extra people added is the series which holds fertility at 1.85 and net migration at zero. Previous work shows that replacement fertility and nil net migration would be even more effective than this series. However, this combination of assumptions has not been used by the ABS.