Federal Labor is defending its guarantee none of its MPs would be caught up the dual-citizenship saga after four were snared by a High Court ruling.
Labor's Susan Lamb, Josh Wilson and Justine Keay are quitting parliament, along with Centre Alliance's Rebekha Sharkie, after the High Court disqualified ACT Labor senator Katy Gallagher.
All of the MPs were deemed British citizens when the writs for the 2016 federal election were issued, which is in breach of the constitution.
Labor leader Bill Shorten, who last year said "we have a strict vetting process, there is no cloud over any of our people", now says he acted in good faith after relying on Labor's legal advice.
Frontbencher Tony Burke says Labor had been relying on the so-called 'reasonable steps' test.
"We now have a precedent that basically knocks out the reasonable steps test," he told Sky News on Thursday.
Mr Burke argued the opposition had tried to refer the MPs as part of package deal with the government, some of whose own MPs were in doubt.
"It's ended up where it has. We're now in by-elections and the resignations will be in today for those members and they're off in their electorates fighting their campaigns," he said.
Meanwhile, Labor is turning the spotlight on Liberal backbencher Jason Falinski, who insists he doesn't hold Polish citizenship.
The MP lodged a new letter from the Polish Embassy on Wednesday.
"I can confirm that according to our records you have never had a Polish passport, or a Polish identity card, and you have never applied through our Embassy to relevant Polish authorities to confirm the possession of Polish citizenship," the ambassador wrote to Mr Falinski.
But Mr Burke says he can't reconcile that with documents held by the National Archives showing Mr Falinski's father and paternal grandparents all declared their nationality as Polish in 1958.
There are now five federal lower-house seat by-elections pending. Four, including three for Labor, are as a result of the citizenship issue and the fifth is due Labor's Tim Hammond leaving his seat 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.