Sort it out immediately, don't wait until Christmas, is Opposition Leader Bill Shorten's message to Prime Minister Malcolm Turnbull after their meeting over the citizenship fiasco today.

Key points: Turnbull may extend Parliament sittings to resolve citizenship saga

Turnbull may extend Parliament sittings to resolve citizenship saga He held talks with Shorten today but the meeting ended without agreement

He held talks with Shorten today but the meeting ended without agreement Mr Shorten wants to make changes to Mr Turnbull's plan

The two leaders held talks in Melbourne about Mr Turnbull's proposal for politicians to have to produce documents to show they are eligible for Parliament and are not dual citizens.

Mr Shorten demanded MPs should have to provide the documents by December 1.

He argued that would leave time for anyone facing further questions to be referred to the High Court for a ruling.

Mr Turnbull proposed a longer time frame that might mean Parliament would have to be brought back for a special sitting in mid-December.

Mr Shorten slammed that idea as a waste of money.

Mr Shorten also wants to change Mr Turnbull's proposal to include showing where politicians' grandparents were born.

The ongoing crisis means both sides of politics are understood to be preparing for the possibility of many by-elections on one day early next year, potentially in February.

The ABC understands the Government believes there are four Labor MPs facing citizenship questions, including Susan Lamb and Justine Keay.

But Coalition MPs are also expected to be potentially in breach of the constitution, including John Alexander.

Both sides of politics are preparing for the possibility that there will have to be a series of by-elections to replace people found to be ineligible.

The Prime Minister's proposal gives all politicians 21 days to produce the documents but Labor said that is not tough enough and might mean some MPs with questionable status are not uncovered until next year.

To avoid the accusation that he is trying to "skate through" the rest of the year, the ABC understands Mr Turnbull is recalling Parliament for an extra week in December.

The Senate is expected to consider Mr Turnbull's plan when it meets next week, but the House of Representatives is not meeting again until later in the month, which means the 21-day period to reveal documents would not expire until around December 18.

It is a frequently used political tactic to threaten to keep parliament sitting until Christmas as a way of piling pressure on to reach an agreement.