"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.
As the Pope arrived in Sydney, Australian Prime Minister Kevin Rudd said he was "an honoured guest among us".
During his trip, the Pope will also meet Aborigine groups, and is likely to repeat an apology made by Pope John Paul to Australia's indigenous people for injustices carried out by Catholic missionaries.
He also told reporters he wanted to wake up consciences, to make politicians and experts respond to environmental issues.
"We have to face up to this great challenge and find the ethical capacity to change the situation of the environment for the good," said Pope Benedict.
But he made it clear he had no intention of addressing any specific political or technical questions about climate change.
The 81-year Pope's medical advisers have taken elaborate steps to protect his health during the gruelling 20-hour flight from Italy, says the BBC's David Willey in Rome.
A special sleeping compartment has been prepared on the Alitalia plane and the German pontiff will spend three full days resting at a Catholic study centre before appearing at the head of a Sydney Harbour flotilla on Thursday.
He was met by Australian prime minister Kevin Rudd on his arrival at Sydney airport.
Pope Benedict will close his trip by presiding over an open-air Mass on 20 July at Sydney's Randwick Racecourse, which is expected to draw hundreds of thousands of pilgrims.
The Pope's visit - his ninth outside Italy - has created controversy with demonstrators vowing to protest against the Church's stand on homosexuality and birth control.
In response, authorities in the state of New South Wales have introduced new laws bringing the threat of heavy fines to anyone causing annoyance to pilgrims.
Civil liberty groups have denounced such sanctions as unnecessary and repugnant.
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