In a further twist in Australia's dual citizenship saga, the country's High Court ruled that Katy Gallagher, an opposition Labor MP in the upper house, was a British citizen when elected in 2016 and therefore barred from standing by the constitution.
Labor's John Wilson (WA), Justine Keay (TAS), and Susan Lamb (QLD) and the Centre Alliance's Rebekha Sharkie (SA) quit within hours of the judgement.
"Consequently, I will resign from the Australian Parliament today and seek re-election at the forthcoming by-election, which I believe the earliest can be the 16th of June and I would hope it would be that date".
Labor already faces a byelection for the seat of Perth, with Tim Hammond announcing last week he would resign for family reasons.
As a senator, Gallagher will be replaced by another candidate from her Labor Party without a fresh election.
Sharkie resigned following the High Court's decision to render Gallagher ineligible to hold office on Wednesday.
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Four Labor MPs were forced out of Parliament yesterday because of citizenship issues, after Mr Shorten had repeatedly declared his party did not have a problem with dual citizens. "I have not served a single day as anything other than an Australian citizen", he said.
On Wednesday the high court ruled that the Labor senator Katy Gallagher was ineligible because she had failed to renounce her British citizenship by the nomination deadline before the 2016 election.
The ABC reports that all of the pollies bar Lamb have successfully renounced their dual citizenships after being elected.
"The decision is clear and doesn't change the law", Mr Porter said. But the Commonwealth says that she was still too late, as the UK Home Office did not registered her renunciation until August, more than a month after the election.
"To the people of the (Australian Capital Territory), I'm very sorry that this disruption has occurred to one of your federal representatives", she said on Wednesday.
Late past year, Prime Minister Malcolm Turnbull called yet another inquiry into Section 44 of the Constitution to see if changes should be made to either make it easier to renounce dual citizenship or whether the requirement should be deleted.