The Australian Constitution can only be changed by a process called a referendum. A referendum is a vote by Australians to decide if they will support or reject a bill (a proposed law) to change the Australian Constitution. A referendum will only be successful if it is supported by a majority of voters in a majority of states, and by a majority of voters across the nation as a whole (a double majority).
Nineteen referendums proposing 44 changes to the Constitution have been held since 1901. However, only eight of these changes have been agreed to by the Australian people (the last was in 1977).
Check out this fact sheet about referendums for more information about how referendums work.