Australia's most senior Catholic George Pell has criticised the departing Pope Benedict, saying his decision to retire could set a precedent which may be a problem for future leaders.

The Archbishop of Sydney was speaking in Rome after the pontiff delivered his final public address in front of tens of thousands of people in St Peter's Square.

Australia's only representative at the forthcoming papal conclave to elect Benedict's successor told AM he stood by earlier comments in a television interview in which he said the Pope was a better theologian than he was a leader.

He said Benedict's decision to step down had destabilised the church and said some of those surrounding the Pope had failed to support him in his ministry.

"He was well aware that this is a break with tradition [and] slightly destabilising," Cardinal Pell, who has been one of Benedict's strongest allies, told AM.

"But he felt that because of his weakness and sickness, which was only too evident today, that he didn't have the strength to lead the church in these demanding times.

"He's as aware as I am of the slight change to the tradition."

He singled out the so-called Vatileaks scandal, which saw the Pope's butler Paolo Gabriele sentenced to jail for stealing classified Vatican documents, as a failure of governance.

"I think the governance is done by people around the Pope and that wasn't always done brilliantly," he said.

"I'm not breaking any ground there, this is said very commonly.

"A change of procedures would have made it more difficult but it's easy to be wiser after the event, it was totally unprecedented.

But he said he still believed Benedict had been a "magnificent teacher".

Earlier the Pope had addressed an estimated 150,000 people, bidding an emotional farewell the day before his departure.

He said he understood the gravity of his decision to resign but that he had done it for the good of the church.

He said his crisis-hit papacy had included moments of joy but also difficulty when "it seemed like the Lord was sleeping".

"There were moments when the waters were choppy and there were headwinds," he said.

"I ask each of you to pray for me and for the new pope.

"I took this step in the full knowledge of its gravity and rarity but with a profound serenity of spirit.

"I was deeply grateful for the understanding, support and prayers of so many of you, not only here in Rome, but also around."

When he finished his speech the crowd, including many red-hatted cardinals, stood to clap.

Cardinal Pell was in the audience for the address and described it as a "beautiful farewell".

He said the papal conclave could get underway within a fortnight.

"There is talk of general congregation happening on Monday [so] a guess would be in 12 to 13 days time," he said.

He said the new pope should be someone who could "maintain the tradition both in faith and in morals" but said they would also need to "lift morale and provide a bit more discipline."