Great question! There have been a couple of changes to the electoral systems used for federal elections over the last hundred and some years. Currently, preferential voting is used to elect members of the House of Representatives and a proportional representation system for the Senate. Before 1949 preferential voting was also used for Senate elections.
At the first election for the federal Parliament in 1901, members of parliament were elected according to the electoral system of each state. From 1903 'first-past-the-post' was the electoral system used for both the House of Representatives and Senate, until preferential voting was introduced in 1918.
The changes to the Australian electoral system are included in the Federal Parliament history timeline.