Pope Benedict XVI has marked the start of a trip to Australia for World Youth Day, a global youth rally billed as the Catholic Woodstock, by saying he will apologise for sexual abuse perpetrated by Australian priests.

Speaking to reporters on the 20-hour flight, the Pope said he planned to examine how the Catholic Church can “prevent, heal and reconcile” the past crimes. “This is the essential content of what we will say as we apologise,” he said.

In April, while in the US, the 81-year-old pontiff apologised for clergy sex abuse and took the unprecedented step of meeting the victims of clerical sex abuse.

About 1,000 Australians turned out to see his plane land today at Sydney airport. They included 150 protesters, campaigning against World Youth Day, which is described by its organisers as the largest youth festival in the world.

Half a million Catholics are expected to attend the closing Mass at Randwick racecourse in Sydney next Sunday. Thousands from around the world have already arrived for the five-day event, but protesters plan to object to the Catholic Church’s stance on homosexuality and contraception and laws passed by the Government of New South Wales banning them from “annoying” World Youth Day pilgrims.

The Pope said he wants to focus on the need to care for the planet, during his visit.

“We must awaken consciences,” he told reporters aboard the papal plane. “We have to face up to this great challenge and find the ethical capacity to change the situation of the environment for the good.”

He said that while a crisis of religion in Europe led some to say God was not necessary for happiness or for creating a better world, this was not so much the case in the US and Australia.

"On the other hand, God is in the hearts of human beings and can never disappear," he said. "There is always a presence of faith. Now that (at) this historical moment we begin to see some belief again, in this sense I am fairly optimistic."

Benedict XVI was met at Sydney airport by the Prime Minister of Australia Kevin Rudd and the Roman Catholic Archbishop of Sydney, the Most Rev George Pell. In an official statement, the Pope said all Australians, especially those who were sick or in difficulties, were in his thoughts. He will now spend three days on a private retreat before the official start of World Youth Day on Tuesday.