There was a moment last Thursday night when blood-curdling horror flashed across Mathias Cormann's face.

He had realised that his best mate Peter Dutton didn't have the numbers to become prime minister after all.

One witness said the normally unflappable, inscrutable political professional had the look of someone who'd gladly commit murder.

Mathias Cormann doesn't like to lose.

Hours after the press conference where he publicly walked away from his leader, Mathias Cormann would realise his intel was bad. ( ABC News: Matt Roberts )

This was a horrible realisation for the Belgian-born powerbroker who had put his honour in the guillotine by walking away from Malcolm Turnbull, just hours after publicly pledging loyalty to the besieged PM.

Senator Cormann had told Turnbull on Wednesday afternoon that he "no longer enjoyed the support of the majority of members in the Liberal Party partyroom". Now he was not so sure.

A man who would normally rely on his own counsel and well-honed instinct, Senator Cormann had allowed himself to be convinced that the Dutton uprising was a dead certainty.

He had been assured by Dutton himself that he had the numbers. Cormann's mistake was to believe him.

Peter Dutton was sure he had the numbers to topple Malcolm Turnbull. ( ABC News: Matt Roberts )

Liberals now point to the incompetence of Dutton's ragtag numbers men: Tony Pasin, Andrew Hastie, Zed Seselja and Michael Sukkar, all of whom are from the so-called "monkeypod" faction, named after the regular Tuesday lunchtime group that meets around a tropical timber table in the ministerial wing.

It was only on Thursday that Senator Cormann began seriously poring through the numbers himself that it hit: his mate Dutton had been relying on bad intel.

The Turnbull camp had been insistent all along that Dutton didn't have the numbers, even if the Prime Minister indicated his own doubts.

About 9:20pm on Thursday, 15 hours before the partyroom meeting, the ABC was told emphatically that Dutton did not have sufficient support.

And it was in this maelstrom of miscalculations by the challenger that Scott Morrison's candidacy emerged. He was the Turnbull candidate by virtue of not being Dutton, and having the best chance of cruelling the coup.

The unlikely victor was Malcolm Turnbull's own Treasurer, Scott Morrison. ( ABC News: Matt Roberts )

They tried to head it off

Anxiety about where Cormann stood in the leadership tumult had been high inside the Turnbull camp — before he, Michaelia Cash and Mitch Fifield announced publicly on Thursday morning they had abandoned the PM.

Sorry, this video has expired Three crucial cabinet ministers withdraw support for Turnbull

On Wednesday, efforts to bind Cormann to team Turnbull had been frantic.

A Turnbull consigliere had sought to conscript Cormann to the team.

And his WA colleague Linda Reynolds was seen having a robust conversation with him, arms rich with gesticulation, outside the Senate shortly before 11am. Her message? Unity.

But Cormann remained aloof.

"If only Mathias had called, we'd have shown him Dutton didn't have the numbers," one Turnbull backer said.

But at 4:30pm on Wednesday, just a few hours after declaring, "I support Malcolm Turnbull as Prime Minister", alongside the Prime Minister and the Treasurer at a media conference, Cormann advised Turnbull he had lost majority support of Liberals.

And it is unclear whether he had forensically checked this assessment before ventilating it to Turnbull.

He offered to resign as Finance Minister, but not before privately acknowledging Turnbull's assessment that his prime ministership was being attacked by "insurgents".

One Turnbull backer says if Cormann had stuck with the PM, Turnbull would've survived the week.

Cormann and followers proved crucial in Friday's spill

Once Cormann absconded to the Dutton camp, Michaelia Cash followed, locking in two others from WA: Ian Goodenough and Slade Brockman.

These were four votes that proved critical in the spill motion on Friday, which was carried narrowly 45-40.

Mathias Cormann and his followers were key to Malcolm Turnbull's downfall. ( ABC News: Matt Roberts )

The pressure was so great that at least two Liberals who did not want change had reluctantly signed the petition to ensure it would get the 43 signatures demanded by Turnbull.

Warren Entsch and Karen Andrews signed, but then voted against the leadership spill motion on Friday — that is, they provided the necessary support for the partyroom to have an emergency meeting but then voted to stick with Turnbull.

Queensland Liberal MP Warren Entsch reluctantly agreed to be the 43rd — and final — signature on the petition for Friday's spill. ( ABC News: Matt Roberts )

One senior Turnbull backer, echoing a widespread view inside the party room, says Turnbull's leadership was cooked as soon as 35 Liberals voted for Peter Dutton in the surprise ballot brought on by the PM last Tuesday.

According to colleagues, Cormann is either the man who was instrumental in Turnbull's destruction or courageous enough to end the damage early.

He is obviously hurting, having played such a key role in bringing down a PM. Some say he is a diminished figure.

Others are adamant he acted appropriately give the awful circumstance.

"In time, people will realise that Mathias did the right thing," one Cabinet minister says.

"The party needed a circuit-breaker and no-one else was prepared to do it.

"He did what he thought was in the best interests of the partyroom. If it hadn't ended for Malcolm on Friday, it would have ended in a fortnight's time."