Katy Gallagher being ruled ineligible likely to lead to byelections in three Labor-held seats and one held by Centre Alliance

The high court will rule on Labor senator Katy Gallagher’s eligibility to sit in parliament on Wednesday morning, a decision that is also likely to indirectly decide the fate of four MPs with dual citizenship issues relying on the same defence.

If Gallagher is ruled ineligible the result is likely to trigger resignations leading to byelections in three Labor-held seats (Braddon in Tasmania, Longman in Queensland and Fremantle in Western Australia) and the Centre Alliance’s sole lower house seat of Mayo, in South Australia.

On Tuesday the manager of opposition business, Tony Burke, said if Labor loses Gallagher’s case it would deal with the result “across the board”, suggesting the opposition will accept it as a precedent for its three MPs.

The prospect of a “super Saturday” string of byelections in the seats – all of which are marginal except Fremantle – shapes up as a key pre-election test for Bill Shorten, who must also defend the seat of Perth after frontbench MP Tim Hammond’s resignation.

Gallagher, Justine Keay, Susan Lamb and Josh Wilson of Labor, as well as the Centre Alliance MP Rebekha Sharkie, all took steps to renounce their dual British citizenship before the nomination date for the 2016 election.

All except Lamb were successful but the renunciations were not processed and did not become effective until after the deadline. Lamb remains a dual citizen because the UK home office would not process the application without a copy of her parents’ birth certificate.

Section 44(i) of the constitution disqualifies people who are subjects or citizens of foreign powers from sitting in parliament.

But in the citizenship seven Re Canavan decision, the high court held that an Australian citizen should not be “irremediably prevented by foreign law from participation in representative government” where they have “taken all steps that are reasonably required by the foreign law to renounce his or her citizenship”.

Gallagher, Sharkie, Wilson, Keay and Lamb have refused to resign from parliament because they believe their circumstances fit the terms of this exemption.

On behalf of Gallagher, the former solicitor general Justin Gleeson argued in the high court that in every case where a person had taken “all steps reasonably required” by foreign law the exception would apply.

At the hearing in March, Labor’s interpretation was met with scepticism by several justices of the high court. Justice Virginia Bell suggested it denuded the requirement that foreign law must “irremediably prevent” renunciation and it was a “radically different” approach to the precedent case of Sykes v Cleary.

The government and Labor both expect the high court to take a strict approach and disqualify Gallagher because she was not “irremediably prevented” from renouncing her British citizenship.

If Gallagher is found ineligible, her Senate seat would be filled in a recount election by the second Labor candidate for the Australian Capital Territory, David Smith.

Katy Gallagher did not do enough to renounce UK citizenship, high court hears Read more

The ACT is set to be given a third lower-house seat in a redistribution, which would give Gallagher the option of contesting the preselection for the No 1 spot to take back her Senate seat at the next election, or shift to the third lower-house seat.

Unless the terms of the judgment against Gallagher give reason to think the other four could escape a similar fate, they are likely to resign.

Malcolm Turnbull has said that Keay and Wilson’s cases “will be determined by the Gallagher case because the facts are essentially the same”.

In December the shadow attorney general, Mark Dreyfus, rejected the view that Gallagher’s case would serve as a precedent, arguing each case might have to be decided on its individual facts.

But on Tuesday Burke told Radio National that Labor would wait for the decision but it if it lost the case it “will obviously have to deal with that across the board”.

In the event Gallagher is ruled eligible to remain in parliament, pressure on the four lower house MPs to resign will dissipate, saving taxpayers several million dollars in byelection costs and Labor the embarrassment of losing three MPs, even if only temporarily.