Section 15 of the Constitution gives the States the power to fill “casual vacancies” in the Senate when a Senator resigns their seat or dies. This has happened quite a lot since Federation; the very first casual vacancy was created in 1903 on the death of Free Trade Senator Frederick Thomas Sargood, who was replaced by another Free Trader, Robert Reid. Since then it’s happened more than a hundred times.

Before 1975, the convention was honoured that a vacancy be filled by a Senator of the same party. There were exceptions, however, and most of those were South Australia’s doing. In 1907, the election of Senator Joseph Vardon was declared void, and the Parliament of South Australia appointed a Labor man, James O’Loghlin, to replace him. O’Loghlin’s appointment was also declared void, and the matter was settled by a special election, which Vardon won. South Australia did it again in 1912, when they successfully appointed Liberal John Shannon to replace Labor’s William Russell. In 1925, the conservatives got revenge on Labor for O’Loghlin by appointing Nationalist Henry Barwell to replace Senator O’Loghlin (since elected in his own right) on his death. They pulled the same stunt again in 1927 (Labor’s McHugh with Nationalist Verran). It also happened in Queensland in 1928, New South Wales in 1931 and again in South Australia in 1931. After this, the convention became firmly established. In 1967, Independent Senator Clive Hannaford was replaced by Liberal Condor Laucke, but since Hannaford had been elected as a Liberal, this didn’t violate the convention.

Everything changed in 1975 when two Coalition state premiers opted to politicise the vacancies and stick it to their favourite target, the Whitlam government. In February, NSW Senator Lionel Murphy, Whitlam’s Attorney-General and Senate leader, resigned in order to become a High Court judge. The NSW Premier, Tom Lewis, refused to appoint a Labor man and instead chose the very long-term Mayor of Albury, Cleaver Bunton. Bunton was an independent in every sense and was no doubt chosen as a means of depriving Whitlam of one crucial vote in a very close Senate. It was ultimately a futile manoeuver, as Bunton ended up voting with the ALP to pass supply in 1975. The same trick, however, was pulled in September with the death of Queensland Labor Senator Bert Miliner. Labor chose Mal Colston (yes, that Mal Colston) to fill the seat, but the government of Joh Bjelke-Petersen chose to stick it to Whitlam again and instead appointed Albert Field, an obscure French polisher. Field was technically a Labor man, but was strongly opposed to Whitlam and conservative in most views. Bjelke-Petersen had demanded three names from which to choose, but when the ALP declined, Joh went ahead and appointed Field, who became a Senator but was expelled from the ALP and sat as an independent. Field’s selection was challenged because of questions over whether he had resigned from the public service in time, and from October, Field was not present in the Senate. The opposition refused to pair him, however, and thus had an extra vote. Bjelke-Petersen’s gamble paid off, the government lost supply, and the rest is history.

After this display, the Fraser government proposed amending the constitution to make sure it didn’t happen again by requiring the appointment to match the affiliation of the departed Senator. The Constitution was altered by referendum in 1977. Since then, the only deviation has also been in 1977, when Liberal Movement Senator Steele Hall resigned. The South Australian Parliament chose Janine Haines of the Australian Democrats to replace him, reasoning that since the LM had been folded into the Democrats, they were the logical replacement.

The process is actually very straightforward. Usually, the party whose seat is vacant simply chooses someone to fill the spot. The exact method they use varies; some hold preselection ballots, others just catapult a candidate hand-picked by the national executive. However they do it, that name is submitted to the relevant State or Territory parliament, who then vote, sitting jointly, to appoint them. The Governor-in-council, which means the state government, can appoint anyone they like as a placeholder if Parliament isn’t sitting. So for instance, if a Queensland Senator resigned or died now, the Bligh Government would choose someone because Parliament has been dissolved. When the parliament reconvened, say after the election, the new parliament would then vote to fill the vacancy. The Senator who fills the vacancy completes the term. Prior to 1977, they served until the next House election and ran in a special Senate election then.

There is no rule saying the state parliament has to accept the party’s nominee; indeed, the Field case could happen again. But it would be a risky thing to do, and the convention has so far been followed in every case.

The one exception to the state parliament choosing was for the ACT in 1981 and 1988. The first time, Senator John Knight died, and because the ACT didn’t yet have self-government, the Representation of Territories Act gave the power to fill the vacancy to the Commonwealth Parliament, which sat jointly to select Margaret Reid. Seven years later, Susan Ryan resigned and the Parliament again met, choosing Bob McMullan. When Reid resigned in 2003, the ACT Legislative Assembly filled the vacancy.

So Arbib’s seat will be filled by the NSW State Parliament in a joint sitting, not by the O’Farrell government or the Labor Party as such. O’Farrell could theoretically appoint someone other than the ALP’s choice and he’d probably get away with it, because he has the rare jewel of an absolute majority in both chambers. But I doubt very much he will. It doesn’t work like that any more, and he’d be crucified for it. Though given the Gillard government’s unpopularity and the vitriol it attracts from the Liberals, I suppose anything’s possible.

Some systems, most notably the ACT and Tasmania, use a ‘countback’ system to fill vacancies. With multi-member seats it’s quite hard (but not impossible) to have a by-election, so the election results are recounted, omitting the winner, and their preferences directed. Whoever would have won had the winner not contested is selected to fill the vacancy. Both those two parliaments have a provision that the new member must come from the same party as the old. In other jurisdictions, like the NSW Legislative Council, the body itself chooses a new member to fill someone’s vacancy. I personally think the countback system is a lot fairer, and would like to see it adopted for the Senate. This would stop party apparachiks getting Senate positions for several years without facing an election.

Final Factoid: Both the Senators for the ACT, Kate Lundy and Gary Humphries, were originally chosen to fill casual vacancies. And each of the people they replaced, Bob McMullan and Margaret Reid, were also chosen to fill vacancies.

So there you go. Journalists and others, please note all the above and stop getting it wrong. Thank you.