One year and one day after a new parliament sits, the AEC calculates the number of seats each state is entitled to. It does this by summing state populations, dividing that number by the number of members for the states (as stated in s24 of the Constitution), and then dividing the population of each state and territory by that divisor. This ensures each state and territory has a fair number of members in the House of Representatives.
A change in the number of Federal electorates in a state is the first of three triggers that mean the AEC must commence a redistribution (redrawing of electoral boundaries). Other triggers include an uneven number of electors across electorates within a state, or seven years having passed since the last redistribution.