George Pell says it would be hard to find another bishop whose actions were as "grave and inexplicable" as those of Ronald Mulkearns who knew about the crimes of pedophile priest Gerald Ridsdale but kept moving him from one parish to another.

In an extraordinary day of evidence to the sex abuse royal commission on Tuesday, Cardinal Pell squarely laid the blame on Bishop Mulkearns - his former mentor - for decades of cover-up which put hundreds of children at risk from Ridsdale.

As Bishop of Ballarat between 1971 and 1997, Mulkearns presided over decisions to move priests, including Ridsdale, between parishes rather than deal with child sexual abuse allegations.

Cardinal Pell told the royal commission Bishop Mulkearns deceived him, lied to him and described his former boss's actions as "reprehensible" and "absolutely extraordinary."

The bishop, 85, is now terminally ill from cancer and in a nursing home. In February he gave evidence to the commission expressing his profound sorrow at his handling of offending clergy.

Cardinal Pell, who was on a council of seven priests advising the bishop in the 1970s and `80s made no direct reference to Bishop Mulkearns' evidence to the commission.

Among the complaints in the 1980s were that Ridsdale had a 14-year-old boy living with him at the presbytery at Mortlake where he was the sole priest. The commission has heard the boy was subject to constant abuse for six months.

Dr Pell steadfastly maintained in evidence over two days that rumours about Ridsdale never came to his attention, and the priests' criminal activity was never mentioned at a consultors' meeting.

In reply to counsel assisting the commission Gail Furness, who asked if he thought Bishop Mulkearns was just one bad apple in the church, Cardinal Pell replied: "Unfortunately I would have to say that I can't nominate another bishop whose actions are so grave and inexplicable," Cardinal Pell said.

"His repeated refusal to act is, I think, absolutely extraordinary."

The cardinal also denied that his answers laying the blame with Bishop Mulkearns were designed to excuse himself from blame.

He said his answers were designed to answer the commission's queries accurately.

Ridsdale's nephew and one of his victims, David Ridsdale, said that in blaming Bishop Mulkearns, Cardinal Pell had thrown "a whole bunch of people under a bus."

Ridsdale has been jailed for 24 years after being convicted of 138 counts of indecent assault and abuse and is eligible for parole in 2019.