A one-punch assault has left a man in hospital suffering a serious head injury in Sydney's north.

A 31-year-old man was waiting at a bus stop in Dee Why on Monday afternoon, when a man got off an arriving bus and after a brief conversation, punched the waiting man in the head.

The punch caused the man to fall and hit his head on the concrete, fracturing his skull.

Later on Monday evening police attended a home on Pittwater Road where they arrested a 19-year-old man who was later released without charge.

AAP

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