MELBOURNE (AFP) - A national inquiry into child sex abuse opened on Wednesday with Prime Minister Julia Gillard warning Australians that they faced "some very uncomfortable truths".

"This is an important moral moment for our nation,' Ms Gillard told ABC radio as the Royal Commission into Institutional Responses to Child Sexual Abuse got under way at the Victorian County Court in Melbourne.

"When I established this royal commission I understood that it was going to require our whole country to stare some very uncomfortable truths in the face," the prime minister said.

Chairman Justice Peter McClellan announced at the opening that at least 5,000 people were expected to seek to appear, although public hearings were not expected to start for several months.

"Part of the task given to us ... is to bear witness, on behalf of the nation, to the abuse and consequential trauma inflicted on many people who have suffered sexual abuse as children," he said.

"For the individuals who have been traumatised, giving an account of their experiences and telling their story can be an important part of the recovery process."

Mr McClellan admitted it was unlikely the commission could complete its work within the timeframe for the delivery of a final report in June 2015. An interim report is due by June 2014.

Ms Gillard, who announced the inquiry in November after a decade of growing pressure, outlined two goals.

"For the survivors of child sexual abuse, I want this to be a moment of healing, for us to say to them as a nation 'we hear you, you're valued and you're believed' because for too long, so many of these survivors have just run into closed doors and closed minds."

"And second, I want the royal commission to provide for us recommendations about the future.

"We've let children down in the past as a country. We need to learn what we can do as a nation to better protect our children in the future."

Australia's most senior Catholic cleric apologised at Christmas to those who "suffered at the hands" of priests and religious teachers.

Child sex abuse allegations have rattled the Catholic Church across the world, particularly in Ireland but also in the United States, Germany and Belgium.