''Where there is a social organisation within the democracy with a proven and self-admitted case of ongoing criminal activity, and it's related to very vulnerable children, the state should intervene,'' he said.

Delivering the annual Law and Justice Oration in Sydney's Parliament House on Wednesday night, Professor Brennan said: ''Clearly, the church itself cannot be left alone to get its house in order. That would be a wrongful invocation of freedom of religion in a pluralist, democratic society.''

His comments echo those made by RMIT professor and former priest Des Cahill to the state inquiry into the church handling of clergy sex abuse.

Professor Brennan said that as politicians decided how to proceed, they needed the help of ''lawyers committed to justice, not lawyers acting primarily to protect the church or to condemn it''.

On Thursday, Professor Brennan said it was too early to say what form state intervention should take, and that the Victorian inquiry and the current inquiry by retired judge Antony Whitlam in New South Wales would give guidance.

However, he was less pessimistic than Professor Cahill on the church's ability to reform itself. ''As we say in the church, 'semper reformanda' (always to be reformed). It's always reforming. Whether it can do it without other assistance, the jury is out,'' he said.

Professor Brennan said in his oration that any problems in the Catholic Church needed to be identified for the good of all citizens, not just Catholics.

''At the moment, there is little more that any Catholic priest can credibly say on this issue in the public square. I make this plea to all lawyers having a commitment to justice. While putting aside any religious prejudice, please contribute fearlessly to the debate.''