Senate vacancy rules, turnover and the party professionalisation of the Senate

Senate casual vacancies are not rare occurrences in Australia.  Since 1901, there have been some 140 such events. The first senate casual vacancy arose within less than 18 months of the establishment of the new upper house, and was triggered by the sudden death of Victorian Senator Sir Fredrick Sargood, a Free Trader, on 2 January 1903.

The founders of the federal Constitution made provision for the likelihood of such untimely departures. Four different types of vacancies are recognised within the Constitution: death; resignation; absence from the parliament without leave; or a finding from the courts that a legislator is ineligible for membership to parliament on constitutional grounds.


The Australian Senate, photo: Wikipedia

The two chambers of parliament, however, deal with these events in different ways. The Constitution specifies that in the event of a vacancy in the House of Representatives a by-election must be conducted under the same electoral system that was used to elect the vacating legislator. In the case of the Senate, the mechanism for selecting the replacement member varies from the election method used to select senators during the usual electoral cycle. In the event of a vacancy, it falls to the state parliament from which the outgoing Senator represents to make the selection.

While the procedure for filling vacancies in the House of Representatives has not changed since the inauguration of the Commonwealth Parliament in 1901, there have been two important adjustments to the vacancy mechanism for the Senate.

Prior to 1977, state parliaments could select any replacement candidate that they cared to appoint. The only formal constitutional constraint on the state legislatures was that the selection had to be by resolution of both houses where the parliament was bicameral (and, of course, the nominee was eligible to sit in parliament). Further, selection decisions of state parliaments were provisional. Incoming senators only held the seat until the next scheduled election regardless of how much unexpired time remained on the departing senator’s term.

However, since 1977 state parliaments are constitutionally bound to appoint persons from the same party as the outgoing senator. Moreover, the incoming senator is entitled to serve the unexpired portion of the outgoing senator’s term.

As many readers will recall, the reason which provoked this change to the Constitution was the actions of two state parliaments in 1975. The Queensland and NSW parliaments appointed replacement ALP senators who were not endorsed by the party.

These decisions critically altered the partisan composition of the Senate that weakened the ability of the then Whitlam ALP government to pass legislation through the Senate. In turn, this created the conditions for the dismissal of the Whitlam government later that year when the Senate deferred the passage of a critical financial measure.

In the aftermath of the crisis, the new federal Liberal Party and National Party coalition government, with the support of the ALP opposition, succeeded in gaining the necessary support at a referendum in 1977 to amend the Constitution by inserting a new section 15.

The new section 15 all but banished the two uncertainties affecting Senate replacements; a casual vacancy could no longer alter the partisan balance in the Senate, and the complications that had arisen from the possibility of a choice by a state government or an election for an additional senator had been removed.

Rule change and section 15.

The important consequence of the rule change is that it has given parties wide discretion on who is to fill a casual vacancy.  Because the replacement costs for parties are low, the rules have served to increase opportunities for negotiated resignations while maintaining the partisan balance in the Senate between elections.

It essentially allows partisan minded senators to surrender their seat prior to the cessation of their term with virtually no political and electoral risk to the parties that sponsored their election to parliament.

As shown in the Table below, in the first period when electoral systems and resignation rules were similar for both chambers, the midterm resignation rate was much the same for the House and the Senate.

In the second period, following the introduction of Proportional Representatiion using the Single Transfer Vote (PR-STV) for Senate elections, the midterm voluntary exit rate for the Senate was only half that of the House of Representatives. PR-STV produced a more finely balanced chamber after 1949, compared to plurality voting (until 1919) or preferential block voting (until 1949) which usually produced large majorities for one party or coalition. The filling of Senate vacancies under PR-STV made casual vacancies potentially risky for parties if state parliaments could not be counted on to replace like with like.

In the third period, following the change to section 15, the midterm resignation rate for the House of Representatives increased by half over the previous period but the Senate rate increased by more than six times to make the rate over fifty percent higher than for the House of Representatives.


The nature of politics in the House of Representatives has been little altered since the establishment of the two major party groupings after 1910, and the role of by-elections for midterm vacancies is unchanged.  But the sequence of rule changes affecting the Senate has had significant consequences for the place of the Senate in the parliamentary process and for the role of party.

After 1977, the changes to replacement rules for voluntary exits from the Senate, coupled with group ticket voting from 1984 have changed the nature of representation in the Senate. John Nethercote has argued that the involvement of the party machine in the manipulation of Senate membership through the use of midterm replacements has weakened the electoral legitimacy of the chamber.  While the analysis by Peter van Onselen underlines the highly partisan members from the major parties to be deployed for partisan and electoral purposes outside the Senate.

It would seem that the changes to these rules, and to the electoral system, have served to strengthen party control over their senators, accelerating not just the professionalisation of the senate but the party professionalisation of the chamber.

Still interested? You can read the full version of this article: Managing midterm vacancies: institutional design and partisan strategy in the Australian parliament, 1901–2013, co-authored with Campbell Sharman, and published in the Australian Journal of Political Science

Narelle Miragliotta is a Senior Lecturer at Monash University