Vacancies are filled though appointment by joint sittings of the relevant state or territory parliament. The party for which the departing member was elected puts forward one candidate to the joint sitting, and the state or territory parliament accepts the one nomination.

There is no convention that a party must put forward multiple candidates. There is no way the parliament can put forward a candidate that is not the accepted nominee of the party whose member retired.

There is only one loophole. The Parliament can fail to appoint anyone. This arose in 1987 when the Tasmanian Parliament failed to appoint a replacement for Labor Senator Don Grimes.

Details of 1987 Tasmanian incident, and other cases of delayed appointments, can be found in the relevant chapter of Odgers Senate Practice.

The requirement that a replacement comes from the same party was inserted in the Constitution in 1977, one of only eight changes ever made to the Constitution. Before 1977, it was only a convention that replacements come from the same party. There were also problems with the pre-1977 provisions because federal governments were able to engineer casual vacancies that altered the number of members elected at the next Senate election.

These problems were highlighted in the three years of the Whitlam government. First there was the Whitlam government’s appointment of DLP Senator Vince Gair as Ambassador to Ireland and the Holy See, a deliberate attempt to engineer an extra vacancy for the 1974 Queensland Senate election.

Then in 1975 there were two breaches of the convention about appointing a replacement Senator from the same party.

First NSW Premier Tom Lewis declined to appoint a Labor replacement on the elevation of Senator Lionel Murphy to the High Court. Lewis instead nominated Albury Mayor Cleaver Bunton, describing him as a ‘political neuter’ when he meant neutral.

Then following the death of Senator Bert Milliner, the Bjelke-Petersen government insisted Labor put forward three names from which the Queensland parliament could choose. Labor declined and nominated Mal Colston, and the Bjelke-Petersen government nominated and elected Pat Field, a furniture polisher by trade and a Labor Party branch member. Field was almost unknown within the Labor Party. He opposed the Whitlam government, was expelled from the Labor Party, but none of this affected his appointment to the Senate.

Due to a court challenge, Field was absent through the events of late 1975, but the absence of Labor’s Bert Milliner proved critical to the Coalition’s blocking of supply and the demise of the Whitlam government.

With the Gair, Bunton and Field affairs in mind, there was general political agreement that the appointment of Senate casual vacancies had to be tightened, hence the 1977 referendum backed by both sides of politics and accepted by the electorate.

However, this has put Senate casual vacancies completely under the control of the central selection procedures of each political party. The games engaged in between the parties in the 1972-5 parliaments have been changed into games within parties over the filling of casual vacancies.

An alternative view is that the Carr appointment reveals that the casual vacancy process can be used to get good quality members into the Senate and cabinet.

But then there are also issues of the democratic legitimacy of the process. For example, Queensland Liberal Senator Santo Santoro was appointed to a casual vacancy in 2002 and served as Minister for Ageing in the Howard government 2006-7. A series of problems with his declaration of financial interests saw him resign from the Ministry in March 2007 and the Senate in April 2007. Santoro spent five years in the Senate, a year as a Minister, and never faced the electorate via a Senate election.

There is no doubt Bob Carr will replace Mark Arbib as a new NSW Labor Senator. It seems highly unlikely that the NSW Parliament will refuse to fill the vacancy. And compared to many of the other casual vacancies over the last 35 years, Carr’s appointment improves the quality of NSW representation in the Senate.

If you want to read more on the issue of casual vacancies, I recommend the article “Senate Vacancies: Casual or Contrived?” by John Nethercote. It outlines some of the problems that have arisen over democratic legitimacy under the Senate casual vacancy rules, and also provides an excellent history of the casual vacancy conventions that applied before 1977.

UPDATE: I’ve been asked several times what happens if an Independent resigns, who fills the vacancy.

First, if the departing Senator had originally been elected for a party, the replacement would be from the original party. So if Labor-defector Mal Colston had been forced from the Senate, his replacement would have been a Labor Senator.

But the more difficult question is an Independent elected as an Independent.

The Constitution states ‘party’, but this was inserted in the Constitution in 1977 before the first registration of parties by the Electoral Commission in 1984. So any constitutional interpetation of the meaning of party could include a broader recognition of political association beyond that defined by the Electoral Act. A candidate elected as an Independent but who can be defined as part of a ‘party’ with some form organisational structure could be recognised as a party.

But what if there was no party structure at all. How could a replacement be determined?

As anyone who filled such a casual vacancy could be challenged in the High Court, the person chosen to fill such a vacancy must be the one best able to withstand a High Court challenge. That means there are no simple rules to fill such a vacancy, it depends on the individual circumstances.

The test case was the appointment of a replacement for Nick Xenophon when he resigned from the South Australian Legislative Council to contest the 2007 federal election. Xenophon had contested the 2006 election at the head of a group of three candidates, the second of whom was elected along with him at the election. When Xenophon resigned, the SA Legislative Council decided on the third person on the ticket to fill his vacancy.

Such a party-like relation would be important in any determination of who should fill a vacancy. However, had there been a rift between Xenophon and his third candidate, such a replacement may not have been appropriate.

Another example was the resignation of Senator Steele Hall in 1977. Hall had been elected as a South Australian Senator for the Liberal Movement, but he had re-joined the Liberal Party. The Liberal Movement had ceased to exist, though a New Liberal Movement had been formed. Most Liberal Movement members had joined the newly formed Australian Democrats, and the SA Parliament eventually settled on Janine Haines as his replacement. She had been third on the Liberal Movement Senate ticket in 1975 and was now an Australian Democrat. She was appointed to the balance of Hall’s term expiring in June 1978, becoming the first Australian Democrat Senator. Note that this appointment was not covered by the new Section 15, but the Dunstan government chose to apply the logic of the new provision.

In summary, there are different factors that come into play in determining who would be the best replacement for an Independent Senator. The Parliament would have to try and determine the most appropriate person, and the one most likely to withstand a challenge.

In the end, another option would be to leave the seat vacant. If only the balance of a term remained, this may be appropriate. If several years remained, and and the balance of power was tight, the state Parliament may be forced to make the most appropriate appointment it could.