During an in-flight press conference, the 81-year-old pontiff said everything possible would be done to prevent a recurrence of Australia's sexual abuse crisis and to promote healing among the victims.
"We have to see what was insufficient in our behaviour and how we can prevent, and heal, reconcile," he said.
"This is the essential content of what we I say . . . to apologise."
Earlier this year, att the start of his U.S. visit too, Benedict had said he was "deeply ashamed" of the abuse scandal and pledged to work to make sure pedophiles do not become priests.
He said that during the 10-day visit to Australia
he would work for "healing and reconciliation with the victims" of sexual abuse by Catholic clergy there "just as I did in the United States."
"It must be clear ... that being a real priest is incompatible with this (sexual abuse) because priests are in the service of our Lord," he said.
Cardinal Pell of Australia said last week that an apology from the pope would be a 'welcome contribution' at the World Youth Day celebrations that open officially on Tuesday.
World Youth Day, begun in Rome in 1986, is held somewhere in the world every three years. Cities bid for it, just as they do for an Olympics.
It draws in the Catholic Church's big guns: the Holy Father and 80 cardinals. But the key participants are young people from over 170 countries, many of whom have had to scrimp and save for years to get to Sydney.
The WYD celebrations have already been partly overshadowed by the launch on Friday of an investigation into sexual abuse allegations against an Australian priest.
The investigation will centre on allegations from former religious education teacher Anthony Jones that a priest, Father Terence Goodall, sexually abused him in 1982.
Cardinal Pell has ordered a church-appointed panel to investigate the claims.
In a 2003 letter obtained by ABC TV's Lateline, Cardinal Pell told Jones an internal report did not support his accusation of attempted aggravated sexual assault, even though the report had in fact accepted all of Jones's allegations.
The issue of sexual abuse of minors by priests has been a major scandal in several countries around the world after local Churches were found to have moved abusers from parish to parish instead of defrocking them or reporting them to authorities.
When he visited the United States last April the pope spoke repeatedly about the "shame" the scandal had wrought on the Church and he met with abuse victims.
Broken Rites, a group which represents abuse victims in Australia, has a list of 107 convictions for sexual abuse but says the real number is higher and only a handful go to court.
World Youth Day coordinator Bishop Anthony Fisher says the Pope wants to make it clear that sexual abuse has no place in the Catholic Church.
"Well I certainly hope that it's part of an ongoing process of our work at doing the right thing in this difficult area and that for some of the individuals who've been hurt, that it will be part of the healing process," he said.
"Many of the victims will be heartened by the Holy Father's frankness about this, his willingness to confront it and his encouragement to the leaders of the church and the members of the church to do the right thing."
But victims say the Catholic Church in Australia continues to cover up abuse by clergy despite issuing an apology for past abuse and compensation. Some plan to protest during the visit.
The NoToPope Coalition has promised to disrupt the celebration. It's an alliance between gays and lesbians who are angered by the church's teachings on sexuality and anarchists disturbed by new police powers rushed through to bolster security at an event costing the taxpayer over 100 million Australian dollars (94 million US dollars).
The NoToPope Coalition intends handing out condoms to pilgrims and marching with anti-Catholic slogans at World Youth Day venues.
World Youth Day officials say they expect more than 200,000 people to turn out for the event.
Benedict will be greeted at Sydney Harbor Thursday by a group of Aborigines and other young people from the Pacific Basin and deliver what is expected to be an important address. In 2001, John Paul issued a formal apology to the indigenous peoples of Australia, New Zealand and the Pacific islands for injustices perpetrated by Catholic missionaries.
The Australia pilgrimage is the longest in his three-year-old papacy and will test the pontiff's stamina.
Although aides say the pope is in fine health, the Vatican appeared to be taking no chances to ensure Benedict is fit for World Youth Day, canceling a weekly public audience this past Wednesday and most other meetings to give him as much rest as possible.
After he succeeded John Paul three years ago, Benedict said he doubted he would make many long trips. But invitations keep coming in from world leaders and officials of his global 1-billion member flock.
He visited Brazil last year, made a pilgrimage to the United States in April and will travel to France in September.
In a message to young Australians at the start of his trip, the pope urged them to look to God for the answers to their deep questions about the meaning of their lives.
"Many young people today lack hope. They are perplexed by the questions that present themselves ever more urgently in a confusing world, and they are often uncertain which way to turn for answers," he said.
"They see poverty and injustice and they long to find solutions. They are challenged by the arguments of those who deny the existence of God and they wonder how to respond. They see great damage done to the natural environment through human greed and they struggle to find ways to live in greater harmony with nature and with one another," he said.