It's been building all week, and now the final act is here.

Key points: Crucial meeting set for 12:00pm

Crucial meeting set for 12:00pm Next prime minister to be revealed by 1:00pm

Next prime minister to be revealed by 1:00pm Fallout likely to include a Cabinet reshuffle and at least one by-election

Prime Minister Malcolm Turnbull surprised his colleagues on Tuesday by vacating his position, prompting a vote at a scheduled party room meeting that he won against Peter Dutton — 48-35.

The threat to his leadership had been building and he thought he could silence it. But it wasn't enough.

Backers switched sides and called for a new vote.

Mr Turnbull has called for a petition from 43 Liberal MPs to prompt a vote and it was delivered late on Friday morning.

The meeting is scheduled for 12:20pm.

Stay up to date with the latest from the Liberal leadership spill. Follow the blog.

Here's how a meeting will play out today.

How does the voting work?

All 85 Liberal MPs will shortly be inside the party room. Even Senator Arthur Sinodinos, currently on sick leave, will be attending.

Mr Turnbull has indicated he will ask the MPs to vote on whether he should step down — through a "spill" motion.

If that vote succeeds, he has claimed he will "treat that as a vote of no confidence". That could mean he resigns as leader and Prime Minister, which would make Nationals' leader Michael McCormack Acting Prime Minister until a new prime minister is appointed by the Governor-General.

At this point, candidates for the leadership will nominate and a vote will commence.

If more than two candidates nominate, then the vote will likely require multiple rounds. If no candidate secures the majority of votes, at the end of each round, the candidate with the lowest number of votes is eliminated.

If the vote reaches a round with only two candidates left, the candidate with the highest number of votes wins.

A similar process is likely to occur for the position of deputy leader.

All eyes have been on Peter Dutton, now a Liberal backbencher, in Parliament this week. ( ABC: Matt Roberts )

When will we know the outcome?

If the spill vote fails, it is likely we will know Malcolm Turnbull remains Prime Minister within 20 minutes of the start of the meeting. This is how long it took to find out the result of the February 2015 spill motion when Tony Abbott retained the prime ministership.

If the spill vote succeeds, and only two candidates nominate, it is likely we will know the winner within 40 minutes. This is how long it took to find out the result in September 2015 when Mr Turnbull ousted Mr Abbott.

If the spill vote succeeds, and more than two candidates nominate, it is likely we will know who will be prime minister in approximately an hour. This is approximately how long it took to decide the leadership between Mr Turnbull, Mr Abbott and Joe Hockey in 2009.

This could be Malcolm Turnbull's final day as prime minister. ( AAP: Lukas Coch )

What will Turnbull do if he loses?

The Prime Minister indicated he would resign from Parliament.

"I've made it very clear that I believe that former prime ministers are best out of the Parliament," he said on Thursday.

"I don't think there's much evidence to suggest that that conclusion is not correct."

However it's not clear exactly when he would resign. Mr Turnbull could cooperate with a leader that remained loyal to him to delay a by-election.

Treasurer Scott Morrison wants to be prime minister. ( ABC News: Matt Roberts )

If there is a by-election in Wentworth, will the Liberals win?

If Mr Turnbull resigns, a by-election will be held in Wentworth, the wealthiest seat in the country, located in eastern Sydney.

There is scope to delay the by-election if a general election looms, however there are no rules that set out the exact circumstances required.

The NSW state election set for March next year may help justify a delay, particularly given a by-election in the home of the deposed prime minister would be uncomfortable for many in the party.

Wentworth is a strong Liberal seat, with a margin of 17 per cent.

Who could become prime minister?

The key contenders are former home affairs minister Peter Dutton, Treasurer Scott Morrison and Foreign Minister Julie Bishop.

Deputy Leader Julie Bishop has declared she wants the top job. ( ABC News: Jed Cooper )

What's all this about Dutton not being eligible for Parliament?

Through a family trust arrangement, Mr Dutton owns childcare centres which have received payments from the Government.

That could trigger a breach of section 44 of the constitution and make him ineligible to be in Parliament.

Advice on whether Mr Dutton is ineligible released on Friday morning found that he was "not incapable" of remaining in Parliament.

However the advice, prepared by Solicitor-General Stephen Donaghue also noted "it is impossible to state the position with certainty".

Backbencher Peter Dutton speaks with colleagues in the Lower House a day after challenging Malcolm Turnbull for his prime ministership. ( ABC: Matt Roberts )

Can Turnbull's political career — or indeed stable government — be salvaged?

Mr Turnbull's best chance to survive was if the petition failed to muster the required number of signatures, but late on Friday morning it had been delivered.

He could survive if the meeting goes ahead, but enough people who signed the petition changed their minds and back him in the initial motion.

This is extremely unlikely. And even then, he has lost the faith of a large proportion of his party and may face another challenge in future.

The likely overhaul of the Government set to occur today will be the catalyst for major change.

A huge Cabinet reshuffle is inevitable, and the Liberals' public meltdown this week has reinforced the divide between Mr Dutton's conservatives and the moderate wing of the party.