Members of the public and an off-duty police officer used improvised tourniquets to stem the bleeding after the man was bitten on the leg at North Shore Beach, near Port Macquarie on the NSW mid-north coast.
Emergency services were called to the scene about 11am on Tuesday after reports of a shark attack.
The man was taken to Port Macquarie Base Hospital in a critical but stable condition.
Beachgoers and a surfer were treating the 23-year-old man when lifeguards arrived from nearby Town Beach, Port Macquarie-Hastings lifeguards said in a statement on social media.
Nearby beaches will be closed for at least 24 hours.
North Shore Beach is isolated and accessible by a dirt road.
There had been several shark sightings to the north of Port Macquarie in preceding days.
A tagged white shark was detected at multiple locations at Sawtell, near Coffs Harbour, on Monday and Tuesday.
Lifesavers evacuated the water at a local beach after the sighting of an unidentified 2.5m shark on Monday.
BiteMetrix, a website that provides surfers with data on increased risk of shark-human interaction, noted there had been a continued increase in white shark activity in the area during the past week.
5°C