PRIME Minister Julia Gillard has announced a sweeping royal commission into child sex abuse that will probe organisations ranging from the Catholic Church and state authorities to the Boy Scouts and sports groups.

The inquiry into institutional responses to abuse will not just look at perpetrators. It will also cover those who were ''complicit'' - for example, in alleged offenders being moved around - or who by ''averting their eyes'' committed acts of omission. It will also look at how police have responded to the problem

Ms Gillard said the allegations that had come to light recently were heartbreaking. ''These are insidious, evil acts to which no child should be subject,'' she said.

The victims deserved the ''most thorough of investigations'', she said. The royal commission was not to impede police investigations or compensation claims.

Ms Gillard said the inquiry would provide victims with the opportunity to speak out if they chose. ''I understand that for some people it can be healing to get the opportunity to tell their story.''

The Prime Minister spoke to Victorian Premier Ted Baillieu and NSW Premier Barry O'Farrell, who have state inquiries under way - both offered co-operation.

She also contacted the Archbishop of Sydney, Cardinal George Pell, to assure him that the commission would not target any one church. Cardinal Pell said later he welcomed Ms Gillard's announcement.

''I believe the air should be cleared and the truth uncovered. We shall co-operate fully with the royal commission,'' he said.

However, he complained about media coverage of the church, saying ''public opinion remains unconvinced that the Catholic Church has dealt adequately with sexual abuse''.

''Ongoing and at times one-sided media coverage has deepened this uncertainty. This is one of the reasons for my support of the royal commission.''

Melbourne Archbishop Denis Hart, chairman of the Australian Catholic Bishops Conference, told The Age he supported the inquiry.

''I stand by resolutely in saying we must clean this matter up,'' he said.

When asked about the Prime Minister's reference to those who had ''averted their eyes'', he said that anyone who had done wrong must face the consequences.

In an earlier statement, the bishops conference admitted there were ''significant problems'' in some dioceses and religious orders, but said ''talk of systemic problems of sexual abuse in the Catholic Church is ill founded and inconsistent with the facts''.

The federal government will talk with the states and institutions about the commission's terms of reference. The inquiry

is likely to have more than one commissioner. Given the huge scope of the inquiry - the commission would be allowed ''to go where it needs to give us a comprehensive response'' - Ms Gillard indicated the government was unlikely to set a firm reporting date. Cabinet ministers had ticked off on the inquiry.

Some within the government would like to see the royal commission a joint federal-state one. An advantage of this would be that it would facilitate access to state-run institutions.

Before Ms Gillard's announcement, Opposition Leader Tony Abbott had backed an inquiry but stressed it should be broad. He said the community ''must have zero tolerance for the sexual abuse of children''.

''Wherever abuse has occurred it must be tackled, and it must be tackled vigorously, openly and transparently,'' he said. ''A lot of terrible things have been done, and a lot of people have suffered deeply.''

After a swathe of allegations of abuse in Catholic institutions in particular, Ms Gillard was under cross-party pressure to act. Chief government whip Joel Fitzgibbon urged a royal commission, as did Labor backbenchers Doug Cameron, Melissa Parke and Stephen Jones. Crossbenchers Tony Windsor, Rob Oakeshott and Nick Xenophon and the Greens also called for action.

Former prime minister Kevin Rudd was more qualified, saying there would be a case for a royal commission if present inquiries found ''institutional resistance'' by the Catholic Church or if more resources were needed to deal with these matters.

Australian Catholic University professor of law and prominent Jesuit priest Father Frank Brennan expressed some doubts about the commission.

''It's so broad that it risks being counterproductive,'' he said, predicting it would be five to 10 times the size of the royal commission into Aboriginal deaths in custody and would probably take five years.

Mr Baillieu welcomed the royal commission, saying the Victorian Parliament's inquiry into the handling of child abuse had demonstrated its value. The commission would provide the opportunity for a national focus on these issues, he said.

The news came as the Catholic Church agreed to provide the Victorian inquiry with access to files as soon as practicably possible.

''The committee is acutely conscious of the deeply personal and private nature of this material and will handle it sensitively,'' a spokeswoman for the inquiry committee said.

State Labor MPs were generally supportive of a more thorough investigation, although some warned that a full-blown singling out of the Catholic Church might be hard to pass the Catholic right in any caucus.

An online petition calling for a royal commission, hosted at change.org, had gathered almost 10,000 signatures.

Detective Chief Inspector Peter Fox of the NSW police, who has been involved in investigating abuse and has alleged cover-ups, said he was stunned that the commission had been called so quickly and was delighted for the victims.

With JESSICA WRIGHT