7.1 At the national level, population change is the result of births, deaths and net overseas migration. At the state/territory level, an extra component of population change exists - net interstate migration. This is the net difference between arrivals to a state/territory from the rest of Australia and departures from that state/territory to the rest of Australia. Interstate migration is therefore an important determinant of population growth and distribution of the states and territories.
7.2 Within Australia there is no requirement for a person who changes their state of usual residence to register their move. Unlike overseas movements, which are recorded at Australia's borders, there are no direct quarterly measure of arrivals and departures between the states and territories. To be able to measure state/territory population change on a quarterly basis estimates of interstate migration are therefore required.
7.3 The Census is one source of information, with people being asked where they lived one year ago and five years ago. However, as the Census is held only every five years, this is insufficient for producing quarterly interstate migration estimates. Another source of data is therefore necessary.
7.4 Over time, the ABS has used a number of administrative (indirect) data sources to produce quarterly estimates of interstate migration, including electoral roll registrations and family allowance payments. Currently, quarterly estimates of interstate migration are modelled using Medicare information in conjunction with Census data and combined with defence force data. The data used by the ABS are information on interstate change of address advised to Medicare Australia and administrative data from the Department of Defence in the case of the military. For more information refer to the interstate migration section of Chapter 9 - Data sources.
7.5 The Medicare-based model used for generating post-censal estimates of interstate migration is largely superseded when new Census information becomes available. For example, every five years, after data from the following Census have been finalised, the modelled estimates are reviewed against, and potentially replaced by, the interstate migration estimates that are calculated from the Census (i.e. rebased to the Census). This is known as the re-derivation of interstate migration.