Section 15 of the Australian Constitution requires the parliament of the relevant state to choose a replacement. This is done in a joint sitting of the upper and lower house (except for Queensland, which has a unicameral parliament). In the event that the state parliament is not in session, the Governor of the state (acting on the advice of the state's executive council) may appoint a replacement senator, but such an appointment lapses if it is not confirmed by a joint sitting of the parliament within 14 days.
Prior to 29 July 1977, the filing of casual vacancies was complex. While senators were elected for a six-year term, people appointed to a casual vacancy only held office until the earlier of the next election for the House of Representatives or the Senate, at which the vacancy would be filled by the electors of the relevant state.[1] It was also an established convention that the state parliament choose (or the governor appoint) a replacement from the same political party as their predecessor, however this convention was not always followed.[n 1]
Between 1980 and 1989, replacement senators in the ACT were chosen by a joint sitting of both houses of the Federal Parliament, under s.9 of the Senate (Representation of Territories) Act 1973. Prior to 1980, replacement senators in the ACT or the NT were to be elected in a by-election, though this never occurred.
^Small resigned from the Senate after he discovered that he held dual Australian-New Zealand dual citizenship. After renouncing his New Zealand citizenship, he was reappointed to the Senate casual vacancy caused by his own resignation.
^Bernardi was elected as a member of the Liberal Party before the start of the 45th Parliament, but resigned from the party and founded the Australian Conservatives during his term. Prior to the start of the 46th Parliament, the Australian Conservatives was dissolved and Bernardi became an Independent.
^ abCarr resigned one month after the 2013 federal election where he was re-elected for a new Senate term that was due to start on 1 July 2014. His resignation resulted in a vacancy in his current term expiring on 30 June 2014 and his new term due to start on 1 July 2014.[3] O'Neill was appointed to fill both casual vacancies in two separate joint sittings of the NSW Parliament on 13 November 2013 and 2 July 2014.
^In the period between the election and her term formally commencing, Ferris worked as a member of parliamentary staff. In order to avoid any argument that she was disqualified for holding "an office of profit under the crown", Ferris resigned and was re-appointed to the vacancy.
^Elected as a member of the ALP, but resigned from the party during his term.
^Elected as a member of the Liberal Movement, but joined the Liberal party on the break-up of the Liberal Movement.
^Haines had also been on the Liberal Movement ticket with Hall in 1974 however she had joined the Democrats on the break-up of the Liberal Movement.
^Elected as a member of Liberal Party, but quit during his term to become an independent.
^ abLiberal Senator Rex Pearson died on 11 September 1961; Liberal member Gordon Davidson was appointed to fill the ensuing vacancy on 28 September. He did not stand at the 9 December 1961 election and his position was won by Liberal candidate Nancy Buttfield. In order to take up the seat with a term finishing on 30 June 1965, Buttfield resigned her pre-existing seat won at the 1955 election and scheduled to expire on 30 June 1962; Davidson was appointed to that position on 8 February 1962.[1]
^The appointment lapsed on 4 August due to lack of confirmation by the South Australian Parliament, but Vardon was reappointed.
^The election of Anti-Socialist senator Joseph Vardon was declared void on 31 May 1907;[4] on 11 July ALP member James O'Loghlin was appointed to replace him. This appointment was declared void on 20 December,[5] and a supplemental election was held in 1908.
List of invalid elections and appointments to the Senate
This is a list of people who have been declared to have been elected or appointed to the Australian Senate that the High Court, sitting as the Court of Disputed Returns, has subsequently declared to be ineligible. Some of these have actually sat in the Senate and participated in proceedings; however, the High Court has held that their presence did not invalidate the proceedings of the Senate.[5]
^South Australian Family First Senator Bob Day resigned on 1 November 2016. On 5 April 2017 the High Court held that Day was "incapable of being chosen" or sitting as a Senator since February 2016 as he had an indirect interest in an agreement with the Commonwealth. Day's seat was filled by Lucy Gichuhi in a recount.[8] Day sat in the Senate between February and 1 November 2016.
^ abRod Culleton was elected as a member of One Nation, but announced his resignation from the party on 18 December 2016. The President of the Senate declared his seat vacant on 11 January 2017 due to his bankruptcy.[9] On 3 February the High Court declared that at the time of the 2016 election he was subject to being sentenced to imprisonment for up to two years and therefore he had never been validly elected.[10] Culleton sat in the Senate between 2 July and 23 December 2016.
^ abThe election was declared void before the commencement of the Senate term
^Heather Hill was a dual citizen of the United Kingdom and Australia, and the High Court declared she was disqualified from election as the citizen of a "foreign power".[12]
^ abRobert Wood was a citizen of the United Kingdom, resident in Australia, however he had never obtained Australian citizenship. The High Court declared as he was not an Australian citizen Wood was disqualified from election.[13] Wood sat in the Senate between 1 July 1987 and 12 May 1988. Irina Dunn was declared elected on a recount and was expelled from the NDP because she did not resign to allow Wood to be re-appointed.
^ abThe election of Anti-Socialist senator Joseph Vardon was declared void on 31 May 1907;[4] on 11 July ALP member James O'Loghlin was appointed to replace him. This appointment was declared void on 20 December,[5] and a supplemental election was held in 1908 which was won by Vardon.