Pell, 79, was cleared of sex abuse charges in his native Australia in April.

His book Prison Journal, published this month, recounts his 13 months in solitary confinement in tiny cells following one of the most divisive trials in the country's history.

In a 90-minute interview with Reuters in his Rome apartment across the street from a Vatican gate, Pell discussed his darkest moments, how his faith kept him from falling into despair, the harm the worldwide sexual abuse scandal had done to the Church and the current state of affairs in the Vatican.

"Look, it was bad, it wasn't like a holiday, but I don't want to exaggerate how difficult that was. But there were many dark moments," he said late on Monday, wearing a black clergyman suit with a silver cross around his neck.

Pope Francis appointed Pell, the former archbishop of Sydney, in 2014 to head the newly-created Secretariat for the Economy and mandated him with cleaning up the Vatican's murky finances.

Pell ran into resistance from some Vatican officials, particularly Italian Cardinal Angelo Becciu, then deputy secretary of state who wanted Vatican departments to continue controlling their own funds.

Becciu forced out external auditors brought in by Pell as well as the Vatican's first auditor general.

In September, Pope Francis fired Becciu, accusing him of nepotism and embezzlement.

Becciu, who also has been caught up in a scandal involving the Vatican's purchase of a luxury property in London, denies all wrongdoing.

Pell went to Australia in 2018 to face charges and never resumed his Vatican duties.

He returned to Rome in September and has met quietly with his successor as treasurer, Jesuit priest Juan Antonio Guerrero Alves.

"I think we are much, much better placed than we were," Pell said of the state of reform of Vatican finances, including new accounting and controls.

"The great challenge that lies before the Vatican is that it's slowly going broke. Now that's a bit of an exaggeration (but) it's slowly happening," he said, adding that he was basing his comments on public information.

Growing Vatican deficits - the 2020 shortfall is expected to be more than 50 million euros ($A82 million) - and a looming deficit of hundreds of millions of euros in the Vatican's pension fund means potential future trouble.

"You can't go on like that forever," Pell said.

"The only thing that I'm keen on is that people, in a very clear-headed way, face up to the situation."

The COVID-19 pandemic has wreaked havoc with Vatican finances, forcing it to dip into reserves and implement tough cost controls.

The Vatican Museums, a once-dependable cash cow, have been closed for months, resulting in a loss of millions of euros in revenues.

Pell said he had no doubt that Guerrero was honest and up to the job.

"What is important is that he continues to have the support of the Pope and that he is not thwarted the way I was thwarted," he said.

He called the London deal "an absolute fiasco, even in terms of competence," with "enormous losses" that will have to be written off.

Vatican magistrates are investigating the deal.

Pell said the lowest point in his prison ordeal was when his first appeal was rejected in August 2019.

"I was down. I was very disappointed. I came to be very cross," he said.

"(But) I said my prayers and got on with things."

Australian Associated Press