A surfer has been seriously injured in a shark attack at Ballina on the New South Wales north coast.

The 20-year-old man was bitten on his left thigh while surfing at Lighthouse Beach.

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He made his own way out of the water and was assisted by other beachgoers.

Emergency services responded to a call for help at 6:25pm (AEDT).

The man was taken to Lismore Base Hospital in a serious but stable condition.

Emergency services remain on site and the beach has been closed for a minimum of 24 hours.