Bases of the constitutional arrangements
The constitutional foundations for composition of the Senate reflect the federal character of the Commonwealth. Arrangements for the Australian Senate correspond with those for the United States Senate in that each state is represented equally irrespective of geographical size or population; and senators are elected for terms of six years. Both Senates are essentially continuing Houses: in Australia half the Senate retires every three years; in the United States, a third of the Senate is elected at each biennial election. A major distinction is, however, that the United States Senate can never be dissolved whereas the Australian Senate may be dissolved in the course of seeking to settle disputes over legislation between the two Houses.
An important innovation in Australia was the requirement that senators should be "directly chosen by the people of the State". Direct election of United States senators was provided in the constitution by an amendment which took effect in 1913, prior to which they were elected by state legislatures.
The innovatory character of Australia's Senate is also illustrated by contrasting it with the Canadian Senate created by the British North America Act 1867. The provinces are not equally represented in the Canadian Senate; and senators are appointed by the national government, initially for life and now until age 75. Composition on this antiquated basis has deprived the Canadian Senate of the legitimacy deriving from popular choice and has meant, in practice, that the Canadian Senate has not contributed either to enhancing the representivity of the Canadian Parliament (the more desirable because of the first-past-the-post method of election used in the House of Commons) nor to assuaging the pressures of Canada's culturally and geographically diverse federation. Prominent proposals for reform of Canada's Senate in recent decades have included equality of representation for provinces and direct election of senators.
The principle of equal representation of the states is vital to the architecture of Australian federalism. It was a necessary inclusion at the time of federation in order to secure popular support for the new Commonwealth in each state especially the smaller states. It ensures that a legislative majority in the Senate is geographically distributed across the Commonwealth and prevents a parliamentary majority being formed from the representatives of the three largest cities and their environs alone. In contemporary Australia it acknowledges that the states continue to be the basis of activity in the nation whether for political, commercial, cultural or sporting purposes. Many organisations in Australia, at the national level, are constituted on the basis of equal state representation or with some modification thereof; this includes the two nation-wide political parties. By contrast, very few nation-wide bodies are organised on the principle of the election and composition of the House of Representatives. Indeed, in Australia's national life, a body such as the House of Representatives is, if not an aberration, at least relatively unusual. This demonstrates that in Australia federalism is organic and not simply a nominal or contrived feature of government and politics.
Constitutional provisions governing composition of the Senate thus remain as valid for Australia in the 21st century as they were in securing support for the Commonwealth in the nation-building final decade of the 19th century.
In addition to senators elected by the people of the states, the Constitution also provides, in section 122, that in respect of territories, the Parliament "may allow the representation of such territory in either House of the Parliament to the extent and on the terms which it thinks fit". Since 1975 the Northern Territory and the Australian Capital Territory have each elected two senators. The particular arrangements for election and terms of territory senators are set out in detail below.
The principles of direct election by the people and equal representation of the states are entrenched in the Constitution and cannot be altered except by means of referendum and with the consent of every state. On the other hand, the principle of choosing senators "by the people of the State, voting ... as one electorate" is susceptible to change by statutory enactment. It is, however, essential to the effectiveness of the Senate as a component of the bicameral Parliament.