Population components prior to September 2016 will not sum to total population growth due to intercensal difference. Components for September 2016 onwards will sum to total population growth as the intercensal difference for this period will not be known until after the 2021 Census.
Intercensal difference is the difference between the preliminary ERP for 30 June in a Census year (carried forward from the previous Census base using births, deaths and migration data) and the final ERP for the same time point (based on the new Census). The intercensal difference by age and sex for each state and territory is spread evenly across the intercensal quarters. Thus intercensal difference acts as a balancing item that when combined with births, deaths and migration equals total population growth for that period.
For more information see Australian Demographic Statistics, Dec 2017 Feature Article: Final rebasing of Australia's population estimates using the 2016 Census.