Police say treacherous conditions could hamper the search, which resumed this morning, for a missing man in waters off southern Lake Macquarie.

The 23-year-old was one of two American tourists swept off the rocks yesterday afternoon at Snapper Point, north of Sydney.

The second 23-year-old man was winched from the water by the region's rescue helicopter and flown to Newcastle's John Hunter Hospital suffering hypothermia.

Chief Inspector Tim Winmill says search conditions were extremely hazardous yeserday, but he is hopeful they will improve today.

"They were very rough and at this stage the prediction is we won't be able to get our police divers in the water, but we'll have that assessed early this morning to see if there's any chance of that changing," he said.