A man is in hospital after being attacked by a shark at a beach in New South Wales' Illawarra region.

Emergency services said they were called to Seven Mile Beach at Gerroa around 1pm following reports of the attack.

It is believed a 24-year-old Sydney man was participating in a surfing lesson when he was attacked.

NSW Ambulance Duty Operations Manager Inspector Jordan Emery said the man was in water up to his waist at the time.

"He felt quite a significant force and lashing against his legs," Mr Emery said.

Bystanders then noticed deep lacerations to the man's right leg and helped him from the water before applying a tourniquet and pressure.

The 24-year-old man suffered serious lacerations to his right leg. ( ABC News )

"They are serious lacerations," Mr Emery said.

"It's very fortunate that bystanders on scene were capable of providing first aid."

The man also suffered a small laceration to his hand and was flown to Sydney's St George Hospital in a stable condition.

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"I should stress that at no time was a shark sighted," Mr Emery said.

"But certainly based on the circumstances around the incident and the injuries that have been sustained, it would appear that this is a shark attack."

NSW Police said the beach was closed after "reports of a shark attack" while they liaised with Surf Life Saving and the Department of Primary Industries (DPI).