NSW Police are appealing for the public's assistance to find 15-year-old Regan Cash, missing from Penshurst since Monday morning.

"Regan Cash, aged 15, was last seen at a home on Centre Street, Penshurst, about 7.30am [on Monday," NSW Police said in a statement on Tuesday.

"She was spoken to via mobile phone about 6.30pm the same day, but when she did not return as expected she was reported missing to officers attached to St George Police Area Command."

Police and Miss Cash's family are concerned about her welfare.

The teen is described as being of Caucasian appearance with a nose piercing, brown hair and a thin build.

"Anyone who sights Regan or has information about her whereabouts, is urged to contact Triple Zero (000) or Crime Stoppers on 1800 333 000," police said.