The search for a swimmer missing off the NSW Mid North Coast will resume tomorrow morning after an "uncomfortable" hours-long operation failed to find him.

The 20-year-old was seen at Shelly Beach in Port Macquarie about 3.40pm today.

Witnesses told police he was seen getting into the water but didn't resurface.

Police, Marine Area Command, the Westpac Rescue Helicopter, NSW Marine Rescue and Surf Life Saving NSW all set out on a search but couldn't find him before nightfall.

Marine Rescue NSW Inspector John Murray the Port Macquarie branch supplied two boats and two jet-skis to the operation, searching the rocks at both ends of Shelly Beach and 5.5 kilometres out into the ocean.

"Conditions weren't favourable, with a two-and-a-half metre swell," he said.

"It

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