A 58-year-old man will undergo surgery after being attacked by a shark at Lake Macquarie.

NSW Ambulance were called to the scene at Yoorala Road just before 6.30pm.

The victim had suffered severe lacerations to his left arm and was flown to John Hunter Hospital in a serious but stable condition.

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The man had been swimming with a 56-year-old woman, who helped pull him to shore.

He benefited from first aid administered by bystanders, which NSW Ambulance Inspector Grahame Rathbone called "potentially lifesaving".

"Bystanders did a great job applying a makeshift tourniquet before paramedic crews arrived," he said.

The man was "conscious and alert" during treatment, Inspector Rathbone said.