There will be five by-elections in four states following a string of resignations from Federal Parliament on Wednesday.

Key points: High Court ruled Labor senator Katy Gallagher ineligible because of British citizenship

High Court ruled Labor senator Katy Gallagher ineligible because of British citizenship Four MPs resigned in wake of the court's decision, all from non-Government parties

Four MPs resigned in wake of the court's decision, all from non-Government parties The fifth by-election is for resignation of Perth MP Tim Hammond last week for family reasons

Four federal MPs have announced their resignation from Parliament after the High Court ruled Labor senator Katy Gallagher ineligible because she did not renounce her British citizenship in time.

Rebekha Sharkie, Josh Wilson, Susan Lamb and Justine Keay announced they were quitting Parliament in the wake of the Gallagher ruling.

There will be a fifth by-election because Labor's Tim Hammond is also resigning, for family reasons.

It is not clear yet when the elections will be held, but there is an expectation of a "Super Saturday" of polls all on the one day — potentially in late June.

Sorry, this video has expired Centre Alliance MP Rebekha Sharkie resigns from Parliament over citizenship

All of the five MPs to quit are from non-Government parties, so unlike previous by-elections in Bennelong and New England late last year, the Government could not lose its one-seat majority.

Opposition Leader Bill Shorten has repeatedly said that Labor's candidates had been through a thorough vetting process to ensure their eligibility.

After his MPs quit in the wake of the High Court ruling, he defended his previous statements, arguing Labor had acted in good faith.

No Coalition MPs have been caught by this High Court ruling, but the Opposition insists that questions remain about the eligibility of some Government MPs, in particular the member for Mackellar, Jason Falinski.

"We are not saying he should leave the Parliament tomorrow, but that one should be referred to the High Court," Labor frontbencher Tony Burke argued.

Mr Falinski has recently received advice from the Polish Ambassador to Australia, Michal Kolodziejski, following questions about his Polish ancestry.

In a letter dated April 27, 2018, the ambassador sets out three ways in which Polish citizenship can be established: with a passport, an identity card or on confirmation by Polish authorities.

"I can confirm that according to our records you have never had a Polish passport, a Polish identity card, and you have never applied through our embassy to relevant Polish authorities to confirm the possession of Polish citizenship or to be bestowed Polish citizenship," the letter reads.

"Unless you submit at least one of the above mentioned documents you are not entitled to the rights or privileges of Polish citizenship, including a Polish passport."

The most marginal seat among the pending by-elections is Longman, held by Labor's Susan Lamb.

Susan Lamb won the Queensland seat of Longman by a small margin. ( ABC News: Matthew Roberts )

Ms Lamb won the seat on Queensland's Sunshine Coast by a margin of less than 1 per cent.

She insisted in Parliament she would stand again.

The ABC understands the Labor party has submitted an application for Ms Lamb to renounce her citizenship.

She had said she could not renounce her citizenship because she did not have a copy of her parents' marriage certificate as she is estranged from her mother.

But the British Home Office has agreed to process the renunciation without it.

The LNP has not yet pre-selected a candidate for Longman, but said it hoped to have pre-selection organised by the end of next week. Former member Wyatt Roy has confirmed he will not be a candidate.

One Nation leader Pauline Hanson has confirmed her party will run a candidate for the seat.

Voters will also be sent back to the polls in Braddon in north-western Tasmania, which Labor's Justine Keay won from Liberal Brett Whiteley in 2016.

Ms Keay had argued she took all the steps she could to renounce her British citizenship, but she has now accepted the High Court decision.

Ms Sharkie, from the Centre Alliance Party — formerly the Nick Xenophon Team — won the seat of Mayo from Liberal Jamie Briggs in 2016.

Her victory in the previously safe Liberal seat was the only Lower House win for the Xenophon Team at the last election.

There will be two by-elections in Western Australia.

Josh Wilson, who won the safe Labor seat of Fremantle in 2016, has also resigned because of the High Court's Gallagher decision.

Josh Wilson also faced questions about his eligibility. ( ABC News: Matthew Roberts )

He was chosen by Labor after it dumped its previous candidate during the last election campaign, which is when he took action to renounce his British citizenship.

The fifth by-election is for the seat of Perth, where Labor's Tim Hammond announced last week he would resign for family reasons.