When Malcolm Turnbull served in the Federal Parliament, he exuded a sense of infallibility that was truly enviable.

As an erudite self-made man who could ooze charm, he was calculated and smart in cutting a business deal, and a positive, polished, self-assured politician.

Self-doubt, it seemed, was never part of his vocabulary.

But his new book, A Bigger Picture, reveals the man some dubbed "Mr Harbourside Mansion" carried a big, dark secret after his first ousting as Liberal leader in 2009.

He was suicidal.

It was an experience foreign to him, one that was completely overwhelming and left him totally blindsided by his own vulnerability.

"I'd never given any thought to my mental health before — mental illness was something others had to worry about," he writes in the book.

"While I'd had periods of real gloom, especially after the defeat of the republican referendum, what I felt enveloping me now was much more serious.

"For the first time in my life, suicidal thoughts started to enter my mind, unbidden and unwanted."

In the weeks that followed, Turnbull writes he fell further into depression as he wrangled with the decision of whether to continue his parliamentary career or quit altogether.

He was prescribed antidepressants, and it prompted him in April 2010 to announce his intention to quit parliament — something he later retracted.

But deciding to quit didn't end his depression. Turnbull said it only amplified his feelings of worthlessness.

"Within hours I felt sure, surer than anything, I'd made the wrong decision."

He felt he had let the party down with his blunders and believing in Godwin Grech — a public servant who concocted a fake email to falsely claim that then prime minister Kevin Rudd was making decisions to give financial advantage to a benefactor.

He also confesses that upon learning he had been deceived, he wanted to apologise to Rudd from the floor of Parliament.

His advisers, and parliamentary colleagues Tony Abbott and Joe Hockey, urged him not to.

Darkness on the page

Tony Abbott and Malcolm Turnbull in 2009. (Alan Porritt : AAP)

On and off during his life, Turnbull has kept a diary — a habit he resumed after losing the leadership in 2009.

He started again with a series of entries simply titled "Darkness", and locked them away with a password he couldn't recall until he was researching his book in 2019.

He experienced his darkest moments after losing the leadership to Tony Abbott in 2009, by one vote, following an almighty split in the party over supporting Labor's plans for an emissions trading scheme.

"I feel at present like a complete and utter failure. I blame myself for losing the leadership, which by the time I lost it had become one of excruciating pain and daily humiliation. The Grech affair had me despise myself for allowing myself to be connected, no matter how innocently, with something as vile as a forged email. Then, having lost the leadership I sank deeper and deeper into depression, couldn't make up my mind whether to stay or go and was finally persuaded to say I would go when it was obvious I should stay. Now I have engineered a rapidly closing window of opportunity where I could backflip and run again and stay in Parliament. "But for what? More humiliation? A rebirth? Unlikely. The answer is the pain will end at some point — suicide is a permanent solution to a temporary (we hope) problem. But frankly, I am thinking about dying every time."

He emerged from the depressive episode a stronger individual. Only a few of his inner circle were aware of how sick he had been.

Turnbull said the experience underlined his determination when he became prime minister to ensure mental health received additional funding. It was also why he departed politics quickly after the failed Dutton coup.

'His treachery was the worst'

Turnbull opens up about his relationship with Mathias Cormann and Scott Morrison in his memoir. (ABC: Matt Roberts)

In the book, the former prime minister expresses his thoughts about colleagues who supported him, abandoned him and actively undermined his leadership.

On Mathias Cormann: "[His] treachery was the worst and most hurtful."

On Peter Dutton: "Views may differ as to whether I overestimated his competence, but certainly I misjudged his character."

On Scott Morrison: "While it's never possible to be 100 per cent certain about these things, I have come to conclude Scott was playing a double game: professing public loyalty to me while at the same time allowing his supporters to undermine me."

"Nobody is selfless, and politicians least of all. You simply can not do the job without strong self-belief. But there are limits, and the boundary between healthy self-confidence and narcissistic egomania can be a fine one," Turnbull writes.

During the tumultuous final week of his leadership, Turnbull writes he had made up his mind to call an election and made an appointment to see the Governor-General at 8:00am on Thursday, August 23, the day before he lost the leadership.

His wife Lucy, and two close advisers, counselled him against triggering a poll, pointing out he'd have to "pay for it again" by running an entirely personal campaign against Labor, and the right wing of his own party.

Avoiding the abyss

The book recounts Turnbull's time as prime minister, including its messy end. (ABC News: Matt Roberts)

He pulled the plug on his career, leaving politics quickly knowing that to avoid the darkness and not fall into the abyss of despair as he had in 2010, he had to get out, ASAP.

That doesn't mean he won't continue contributing from the sidelines; he says he will maintain the most important title in our democracy: an Australian citizen, with all the responsibility and opportunity that entails.

Many will rip through the 698-page memoir to gain insight into Turnbull's achievements, regrets, the behind-the-scenes wheeling and dealing on policies like Gonski 2.0, his withering assessments about the prime minister he deposed — no spoiler alert needed, he still loathes Abbott — and frank judgments about the man who replaced him.

But at a time when political intrigue seems, at best, like a frivolous parlour game compared to the high-stakes consequences of COVID-19, the confessions about his depression and contemplation of self-harm will leave a lasting legacy.