A desperate search is underway for a missing swimmer at a beach in northern NSW.

The woman, believed to be aged in her 20s, was seen entering the water at Pottsville Beach, Pottsville, about 7am on Tuesday but failed to resurface.

Police said a multi-agency search and rescue was launched shortly after.

“Police with assistance from Marine Area Command, the Westpac Rescue Helicopter, and Surf Life Saving NSW, have commenced a multi-agency search to locate the woman.

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“She is described as being of Caucasian appearance, of thin build, with light brown shoulder-length hair in a ponytail, and was wearing cream-coloured shorts.”

Anyone with information about the whereabouts of the woman, or anyone who matches the woman’s description at Pottsville Beach, is urged to contact police.