A rescue operation is under way after a teenager was reportedly swept off rocks and sucked into a notorious blowhole in Sydney's north.

Emergency services were called to the Warriewood blowhole about 1.15pm after a bystander heard screams for help and called Triple Zero.

9News understands the teenage boy was playing near the rocks at the blowhole when he was knocked into rocky waters below.

READ MORE: Young woman drowns at famous Aussie swimming hole

Rescuers scaled a steep cliff to reach the boy, aged in his early teens, at the base of a cave below.

It's understood he's suffered multiple abrasions, including cuts to his legs and a head injury.

The boy is expected to be winched from the cave to the top of a cliff shortly.

More to come.

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