REBEKHA SHARKIE CITIZENSHIP PRESSER
The citizenship crisis has claimed four more MPs, who quit vowing to recontest their seats after the High Court disqualified Labor senator Katy Gallagher.
Labor's Susan Lamb, Josh Wilson and Justine Keay signalled on Wednesday they would quit, along with Centre Alliance's Rebekha Sharkie, sparking fresh elections in their seats.
All five MPs were British citizens when the writs for the 2016 election were issued, which the High Court ruled made Ms Gallagher ineligible.
Attorney-General Christian Porter demanded the four lower house MPs resign after the ruling, and one-by-one they announced their decisions to quit and run again.
Five by-elections are now due, with Labor's Tim Hammond also set to quit his Perth seat on Friday for family reasons.
They are likely to be held on the same day, with June 16 the earliest possible date for a by-election Super Saturday.
Ms Lamb plans to run again in the Queensland seat of Longman, but she is still technically a British citizen.
"She's going to renounce her citizenship (before nominations close)," Opposition Leader Bill Shorten told reporters, even though he said he was unaware whether she had the documents required to do so.
Longman could be a tough by-election for Labor which holds the seat by the razor thin margin of 0.8 per cent.
Mr Wilson said it had been a "difficult process" but he was ready for a by-election in Fremantle.
Ms Keay will run again in Braddon, but it won't be against former senator Jacqui Lambie, who ruled out a tilt at the Tasmanian seat.
The resignations of the four lower house MPs will take effect from Friday.
"I apologise to the people of Mayo for this turn of events," Ms Sharkie told reporters.
The government accused Labor of failing to act on their in-doubt MPs earlier and falsely saying the party's checks of candidate backgrounds were thorough.
Mr Shorten, who last year said "We have a strict vetting process. There is no cloud over any of our people", said he always acted in good faith, relying on Labor's legal advice, but the court had set a new precedent in the Gallagher case.
Mr Porter said the court had not set a precedent, but rather gave a "crisp and crystal clear clarification of the law" as it stated in the case of government minister Matt Canavan last year.
Ms Gallagher, who represents the ACT, argued she took every possible step to renounce her British ties before the 2016 election.
But the High Court found that despite Ms Gallagher's efforts to renounce her citizenship, she was not eligible under section 44 of the constitution to sit in parliament.
"The fact is that she remained a British citizen under the law of the United Kingdom," the High Court decided.
She could re-enter parliament through a newly-created lower seat in the ACT, and is expected to be replaced in the Senate by fellow Labor candidate David Smith.
A government spokesman told AAP there were no doubts over coalition MPs Julia Banks, Jason Falinski, Alex Hawke and Nola Marino, who Labor sought to refer to the High Court last year.
The spokesman said they had all been cleared of dual citizenship before nominations closed.