Beaches in northern NSW remain closed after a Japanese surfer had his legs bitten off in a fatal shark attack on Monday.

Ballina beaches have been closed since the attack but recent shark sightings have prompted lifeguards to keep swimmers out of the water.

A four-metre shark and a three-metre shark were spotted in waters near Ballina late on Wednesday afternoon.

Surf Life Saving NSW said on Thursday Ballina beaches remained closed until further notice as a precaution.

Tadashi Nakahara, 41, was bitten by a 3-4 metre great white shark while surfing at Shelly Beach on Monday morning.

Rescuers took Mr Nakahara to shore and performed CPR but he died on the beach.

It's the fourth fatal shark attack in Australia in five months.

Experts have said warm water and runoff from rainfall are attracting small fish to the area which, in turn, attract sharks.

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Northern NSW beaches remain closed