A convicted child molester has been prevented from flying overseas from Sydney Airport under new laws aimed at keeping Australian pedophiles out of Southeast Asian sex tourist markets.

Laws that took effect on Wednesday prevent 20,000 convicted pedophiles listed on the Australian child sex offender register from leaving the country except for specific purposes approved by law enforcement agencies.

Foreign Minister Julie Bishop would not say where the pedophile stopped in Sydney was headed. He is being questioned by police.

It is now a federal crime punishable by up to five years in prison for registered child molesters to leave the country without law enforcement agencies' permission.

Australian pedophiles are notorious for taking inexpensive vacations to nearby Southeast Asian and Pacific island countries to abuse children.