Why are some key features of Australia’s system of government not mentioned in the Constitution?

Some features of Australia’s system of government are not mentioned in the Australian Constitution because they were expected to operate through established traditions (known as conventions) or to give Parliament the ability to make changes over time without needing to formally change the Constitution.

For example, the Prime Minister and the Cabinet are not mentioned in the Constitution. While central to the way the Australian Government works, the Prime Minister and Cabinet operate by convention. This is similar to the British system from which they were based. The Constitution does not say that the Government is the party or coalition of parties with the support of the majority of members elected to the House of Representatives. However this has evolved as a necessary practice for the governing of Australia, which also has its origins in British conventions.

Another example is that, while the Constitution says that senators and members of the House of Representatives must be elected by the people, it does not say how voting will work or how elections should be run. This has allowed Parliament to update the voting system over time. One change was the introduction of preferential voting for the House of Representatives in 1918. Another was lowering the voting age from 21 to 18 years old in 1973. These changes were made by Parliament creating laws, rather than changing the Constitution, which means the system can be updated without needing a referendum.