Four Australian members of Parliament resigned Wednesday after revealing they held dual citizenship, bringing the number of lawmakers forced to vacate their seats because of split national loyalties to 15 in less than a year. The fifth MP, Rebekha Sharkie, is part of minor party Centre Alliance.
The decision on Wednesday appeared to leave no room for those MPs to argue they were eligible under Section 44, with the court saying the case turned "upon "one issue" - whether Ms Gallagher was "irremediably prevented by foreign law" from renouncing her dual citizenship".
There will be two by-elections in Western Australia.
That decision prompted four other lawmakers in similar predicaments to Gallagher to resign, triggering by-elections for their lower-house seats.
Opposition Leader Bill Shorten gave a "rolled gold" guarantee a year ago that no Labor MPs would be caught up in the dual citizenship saga.
He said the three Labor MPs had acted in good faith and was confident they would be eligible to contest the upcoming byelections.
Gallagher was aware of her British passport as her father was born in England, so the High Court ruled that she had not taken "all reasonable steps" to relinquish her British links.More news: The rematch did not happen. Bellew knocked out hay
More news: State joins lawsuit challenging DACA program
More news: SEC Files Action to Compel Jay-Z's Testimony in Clothing Company Investigation
The court found that opposition Senator Katy Gallagher was a British citizen at the time of her election, despite her attempts to renounce it, and ineligible for office, the ABC said.
Attorney General Christian Porter says a new High Court ruling means four sitting MPs must resign before the end of the day.
Porter said for Shorten to claim it was a reinterpretation was "talking absolute rubbish".
University of Sydney constitutional expert Anne Twomey said the crux of the court's decision was that the test of someone having taken reasonable steps to renounce their foreign citizenship - the argument on which Gallagher relied - applied only when the country actually or effectively would not let the person renounce. My paperwork was lodged and received by the UK Home Office before the election was called.
The court ruled unanimously on Wednesday that the senator was disqualified.
"Based on the recent High Court judgement, I will now take urgent legal advice", she said.