On 31 August 2016 state senators were divided into two classes: short-term senators whose terms expire on 30 June 2019, and long-term senators whose terms expire on 30 June 2022. These classes were allocated according to the order of election.
Senator Conroy resigned as a Senator for Victoria on 30 September 2016, and the Victorian Parliament selected Senator Kitching to fill the vacancy on 25 October 2016.
On 7 November 2016 under section 376 of the Commonwealth Electoral Act 1918, the Senate referred two matters to the High Court. Both involved the qualification of senators under section 44 of the Constitution. The first involved a possible pecuniary interest relating to the lease for an electoral office for former Senator Day, who had resigned as a senator for South Australia on 1 November. The second matter related to Senator Culleton and involved a conviction for larceny which was subsequently annulled, but had stood throughout the election period.
The Court of Disputed Returns found that neither senator had been validly elected and in each instance a recount of ballots was ordered. As a result Peter Georgiou from Pauline Hanson's One Nation Party was elected to replace Senator Culleton, and Lucy Gichuhi from Family First was elected to replace Senator Day. Senator Gichuhi later indicated that she would sit as an independent senator. On 5 February 2018, Senator Gichuhi joined the Liberal Party.
On 7 February 2017, Senator Bernardi announced his resignation from the Liberal Party and now sits as an Australian Conservatives senator.
Senators Ludlam and Waters of the Australian Greens announced their resignations, on 14 and 18 July 2017 respectively, due to dual citizenship (see section 44(i) of the Constitution). Both matters, as well as matters regarding the eligibility of Senators Canavan, Nash, Roberts and Xenophon were referred to the Court of Disputed Returns. On 27 October 2017, the Court found that Senators Canavan and Xenophon were eligible to have been elected to the Senate and Scott Ludlam, Larissa Waters, the Hon. Fiona Nash and Malcolm Roberts were ineligible.
Senator Back resigned as a Senator for Western Australia on 31 July 2017, and Senator Brockman was selected by the WA Parliament to fill the vacancy on 16 August 2017.
On 10 November 2017 Senators Jordon Steele-John, Andrew Bartlett and Fraser Anning were declared elected by the High Court in place of Scott Ludlam, Larissa Waters and Malcolm Roberts, respectively. Senator Jim Molan was declared elected on 22 December 2017 in place of the Hon. Fiona Nash.
On 15 January 2018, Senator Anning indicated that he would sit as an independent senator. On 7 June 2018, he advised that he had joined Katter's Australian Party with effect from 4 June. On 26 November 2018, Senator Anning indicated that he would again sit as an independent senator.
Senator Nick Xenophon resigned his place as a senator for South Australia on 31 October and Rex Patrick was selected by the South Australian Parliament to fill the vacancy on 14 November 2017.
Senator the Hon. Stephen Parry resigned as President of the Senate and as a senator for Tasmania on 2 November 2017. Senator Lambie resigned as a senator for Tasmania on 14 November, and Senator Kakoschke-Moore resigned as a senator for South Australia on 22 November 2017. On 9 February 2018, Senators Richard Colbeck and Steve Martin were declared elected in place of Stephen Parry and Jacqui Lambie, respectively. On 13 February 2018, Senator Martin indicated that he would sit as an independent senator. On 30 May 2018, Senator Martin joined The Nationals. Senator Tim Storer was declared elected in place of Skye Kakoschke-Moore on 16 February 2018, and advised that he would sit as an independent senator on 26 February 2018.
Senator Dastyari resigned as a Senator for New South Wales on 25 January 2018, and Senator Kristina Keneally was selected by the NSW Parliament to fill the vacancy on 14 February 2018.
Senator Brandis resigned as a Senator for Queensland on 7 February 2018. Senator Amanda Stoker was selected by the Queensland Parliament to fill the vacancy on 21 March 2018.
On 9 May 2018, the Court of Disputed Returns found that Senator Katy Gallagher was ineligible to have been elected. A special count of ballots was held to fill the vacancy, and David Smith was declared elected on 23 May 2018.
On 18 June 2018, Senator Burston announced his resignation from Pauline Hanson's One Nation, and advised that he would sit as an independent senator. On 19 June 2018, Senator Burston advised the Senate that he had joined the United Australia Party.
On 15 August 2018, Senator Rhiannon resigned as a Senator for New South Wales and Senator Mehreen Faruqi was selected by the New South Wales Parliament to fill the vacancy. Senator Bartlett resigned as a Senator for Queensland on 27 August 2018. The Queensland Parliament selected Senator Larissa Waters to fill the vacancy on 6 September 2018.
Senator Bushby resigned as a Senator for Tasmania on 21 January 2019. On 6 March 2019 Senator Wendy Askew was appointed by the Governor of Tasmania to fill the vacancy. Under section 15 of the Constitution, the governor of a state may appoint a person to fill a casual vacancy when the state parliament is not formally in session. The appointment was confirmed by a joint sitting of the Tasmanian Parliament on 20 March 2019.
Senator Collins resigned as a Senator for Victoria on 15 February 2019. The Victorian Parliament selected Senator Raff Ciccone to fill the vacancy on 6 March 2019.
Senator Leyonhjelm resigned as a Senator for New South Wales on 1 March 2019. Under section 15 of the Constitution, the Governor of New South Wales appointed Senator Duncan Spender to fill the vacancy. The appointment must be confirmed by a joint sitting of the New South Wales Parliament when it resumes.
On 11 April 2019, Senator David Smith resigned as a Senator for the Australian Capital Territory.