The Australian Constitution can only be changed in 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 8 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.