Labor and the Greens likely to gain Senate seats at the election

As well as the election for the full House of Representatives, there will be an election on May 3 for 40 of the 76 senators. The 72 state senators have six-year terms, with half of them up for election every three years. The four territory senators are all up for election with each House election.

In a double dissolution election, all senators are up for election, but this election won’t be a double dissolution. State senators elected at this election will begin their six-year term on July 1.

The six states are entitled to equal representation in the Senate, so each state has 12 senators. This system greatly overrepresents Tasmania relative to its population. The ACT and Northern Territory have two senators each.

Senators are elected by proportional representation in their jurisdictions with preferences. At a half-Senate election, with six senators in each state up for election, a quota is one-seventh of the vote, or 14.3%. For the territories, a quota is one-third or 33.3%. Half a quota on primary votes (7.1% in a state) is usually enough to give a party a reasonable chance of election.

Voters will be instructed to number at least six boxes above the line or 12 below the line, but only one box above the line or six below is needed for a formal vote. Preferences are voter-directed, with the group ticket voting system scrapped before the 2016 election. Owing to exhausted votes, the final seats in states are likely to be filled on less than a full quota.

To become law, legislation must pass both the House and Senate in the same form. With the Senate’s proportional representation, it’s very unlikely to be under government control, so governments need to negotiate with other parties to pass their legislation through the Senate.

Party standings and seats up for election

The Coalition holds 30 of the 76 total senators, Labor 25, the Greens 11, One Nation two, the Jacqui Lambie Network (JLN) one, the United Australia Party (UAP) one and all others six.

During this term, Lidia Thorpe defected from the Greens, Fatima Payman from Labor and Tammy Tyrrell from JLN. As all three are state senators who were last elected in 2022, none will be up for election until 2028. The Coalition also lost two senators to defections (Gerard Rennick and David Van) – both were last elected in 2019 and will be up for election this year.

Other than the ACT and NT seats, the seats up for election were last up in 2019. At that election, the Coalition won 17 of the 36 state senators, Labor 11, the Greens six, One Nation one and Jacqui Lambie one.

The Coalition and One Nation combined won four of the six Queensland senators. In Tasmania, Labor and the Greens won three seats, the Liberals two and Lambie one. All other states were tied between the left and right blocs.

In the 2022 election, Labor and the Greens combined won four of the six Western Australian senators, Tasmania once again went three Labor and Greens, two Liberals and Tyrrell from JLN, and the other states were tied between the left and right. The state senators elected in 2022 are not up for election.

The four territory senators elected in 2022 will be up for election. In 2022, the ACT split one Labor and one for independent David Pocock, the first time its senators had not split one Labor, one Liberal. The NT is expected to once again be one Labor, one Country Liberal Party.

Here is a table of Senate seats up for election in 2025. I have ignored the defections of Rennick and Van from the Coalition in Queensland and Victoria. The good news for the left is that only Pocock’s ACT seat looks winnable for the right.