A helicopter search and rescue operation for two kayakers believed to be missing in treacherous water off Sydney’s northern beaches has been called off for the night.


A witness reported seeing two kayakers off Long Reef headland this afternoon who appeared to be in distress, triggering a search and rescue operation.


But two sweeps of the coastline by the a rescue helicopter found nothing, and no reports of missing persons have yet been received by police.


Stephen Leahy, chief executive of the Westpac Life Saver Helicopter Rescue Service, there was a possibility the sighting may have been a "false alarm", but he said he did not doubt the good intentions of the witness.


According to Mr Leahy, the witness reported seeing one of the kayakers come out of their vessel and swim towards the other, with both kayakers then swept further out to sea.


Sydney was hit with gale force winds today and conditions have been difficult, with choppy water and heavy sea mist.


The Westpac helicopter conducted two trips, sweeping the northern beaches from North Head to Palm Beach, up to 10 kilometers out to sea.


The crew used a thermo-imaging camera to try to locate the kayakers.


NSW police will continue investigations in to the reports.


A spokeswoman for police media said there were a number of paddleboarders in the area this afternoon and it was possible the witness could have mistaken some paddleboarders for kayakers.