Hi, thanks for your question
The increases in the number of senators to 60 in 1948 and then to 76 in 1983 were triggered by significant increases to the number of members of the House of Representatives. In 1948, the House of Representatives grew from 75 members to 121. That number gradually rose over the decades until 1983, when it jumped to 148.
Members of the House of Representatives are voted in by electorates, which are areas in Australia which have approximately the same number of voters. As Australia’s population increases it is necessary to create new electorates in order to maintain an equal and manageable number of people in each electorate. This results in an increase in the number of members in the House of Representatives.
Section 24 of the Australian Constitution says the number of senators needs to align to the number of members in the House of Representatives. It states that 'the number of members of the House of Representatives must be twice the number of senators, or as near as practicable.' This is sometime called the nexus provision and was included to prevent the House of Representatives becoming disproportionately larger than the Senate.
For further information about other important changes in Australia’s Parliament visit our Federal Parliament history timeline.