This challenge will test the extent of politicisation of the High Court

Independent senator Bob Day has launched a High Court challenge to the Turnbull government’s senate voting reforms, which threaten his and other crossbench senators’ seats at the next election.

The changes, which aim to prevent minor parties from using preference deals to turn a small primary vote into a seat in Parliament, were passed last week after a marathon 40-hour debate, with the support of the Greens and independent senator Nick Xenophon, paving the way for Prime Minister Malcolm Turnbull’s double dissolution announcement on Monday.

Senator Day asked the High Court late on Tuesday to declare that the changes were invalid and to prevent the South Australian electoral officer and the federal government from issuing the new ballot papers for the next Senate election, to be held on July 2 if the Senate fails to pass the Coalition’s key industrial reforms.

.The legislation passed parliament after a marathon Senate sitting on Friday amid speculation the federal government will soon call a July 2 double-dissolution election, clearing out both chambers.

Family First senator Bob Day is behind the legal challenge but Liberal Democrats senator David Leyonhjelm is also supporting the move.

Although it voted against the changes, and has mocked the reforms, the Labor party is not expected to formally join the legal action.

Senator Day said he thought the challenge had a 50-50 chance of success.

Senator Bob Day and Senator David Leyonhjelm Pic: Andrew Meares

"The Liberal Party, Nationals, Greens and Nick Xenophon teamed up to get rid of independent senators and minor parties. We think that is undemocratic," he said.

He expected other crossbenchers​ to join the action, although Senator Jacqui Lambie declined to support the bid on Saturday. The Motoring Enthusiasts Party’s representative, Senator Ricky Muir – who won his seat with less than 0.5 per cent of the primary vote – also said he would not be joining the challenge.

The basis for the challenge is that 3 million voters will be disenfranchised by the laws. Senator Day is also seeking an injunction from the High Court to intervene before the election to decide whether the electoral changes are constitutionally valid.

High Court Chief Justice Robert French will preside over the case’s first directions hearing on Thursday in Perth via video link to Sydney, where barristers will appear for Senator Day and the defendants, the South Australian electoral officer and the Commonwealth of Australia.

Liberal Democrat Senator David Leyonhjelm has provided some financial support for the case. The changes allow voters to number six preferences above the line, abolish individual and group voting tickets and allow party logos to be printed on ballot papers. It will also allow those who choose to vote below the line by candidate to indicate only their first 12 preferences when previously all candidates – in some cases more than 100 – had to be ranked.

Constitutional law experts UNSW Professor George Williams and Australian Catholic University vice-chancellor Greg Craven have said the challenge is likely to fail because the High Court had made clear the government could use a range of electoral systems to satisfy direct election.