Pope Francis fires head of child sexual abuse investigations office.

Pope Francis has sacked the head of the Vatican office that handles sex abuse cases, just days after he released Australian cardinal George Pell to return home to face charges of historical sexual assault.

The developments underscored how the Catholic Church’s sex abuse crisis has caught up with Francis, threatening to tarnish his legacy over a series of questionable appointments, decisions and oversights in his four-year papacy.

Francis on Saturday declined to renew the mandate of German Cardinal Gerhard Mueller as prefect of the Congregation for the Doctrine of the Faith, the Vatican office that processes and evaluates all cases of priests accused of raping or molesting minors.

Francis named Mueller’s deputy, Monsignor Luis Ladaria Ferrer, a Spanish Jesuit, to run the powerful office instead.

During Mueller’s five-year term, the congregation amassed a 2000-case backlog and came under blistering criticism from Irish abuse survivor Marie Collins, who had been tapped by Francis in 2014 to advise the church on caring for abuse victims and protecting children from paedophile priests.

Collins resigned from the papal commission in March, citing the “unacceptable” level of resistance from Mueller’s office to heeding the commission’s proposals.

In May, Francis said her criticism of the slow pace in processing abuse cases was justified and announced he was adding more staff to handle the overload.

Earlier this year he also named Cardinal Sean O'Malley as a member of the congregation in hopes of ensuring better cooperation.

Mueller's ouster was the second major Vatican shake-up this week.

On Thursday, Francis granted Cardinal Pell, another Vatican hardliner, a leave of absence to return to Australia to face multiple charges of historical sexual assault.

The cardinal has strongly denied the charges.

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Francis has come under criticism for having named him to the powerful position of the Vatican's money czar in 2014 in the first place, given that accusations of wrongdoing had dogged him even then.

Pell has been widely denounced at home for mishandling abuse cases while he was a bishop and of having treated victims harshly in seeking to protect the church from abuse-related civil litigation.

Mueller and Pell were two most powerful cardinals in the Vatican, after the Vatican secretary of state, Cardinal Pietro Parolin.

The week's events could be seen as an attempt by Francis to turn the page, given his legacy has already been sullied by repeated failings to make good on his "zero tolerance" pledge for abuse.