That is an interesting question, Abigail. Section 128 of the Australian Constitution says the Constitution can only be changed if a majority of voters in a majority of states, and a majority of all voters around Australia agree in a referendum. The Northern Territory and the Australian Capital Territory did not exist when the Constitution was drafted, so were not included in this provision. However, citizens in the 2 territories are counted in the national majority.
Learn more about referendums with this fact sheet.