Thanks for your question.
The composition of the Senate is explained in Section 7 of the Australian Constitution and includes ‘equal representation of the several Original States’. There is no mention of territories having Senate representation and, until 1975, no territory had any senators.
However, Section 122 of the Constitution states that the Australian Parliament may allow territory representation ‘in either House of the Parliament to the extent and on the terms which it thinks fit’. This means the Australian Parliament can allow a territory to elect senators and determine how many senators, if any, a territory may have. Territories do not have to be equally represented, unlike the six original states.
In 1968 a bill to allow territory senators was introduced to the Parliament. It proposed 2 senators be elected each from the Northern Territory and the Australian Capital Territory. This bill was debated, but not agreed to, in 1968 and again in 1970. It was also the subject of 2 High Court challenges about whether the Constitution allowed for territory senators. Finally, a joint sitting of the Australian Parliament passed the bill on 6 August 1974.
There are now 76 senators in total: 12 elected from each of the 6 Australian states and 2 from each of the two mainland territories.