A diver has captured "incredible" footage of a group of sharks, including great whites, feeding off the carcass of a dead sperm whale in waters off Coffs Harbour, on the New South Wales mid north coast.

Brett Vercoe and his wife were on a diving trip yesterday morning when they came across the whale carcass.

He said he saw at least five large sharks feeding on the dead whale.

"In a short period, we saw a number of sharks circling around [the whale]," he said.

"After 10 or 15 minutes it was quite obvious there were at least five sharks - three white pointers, up to about 4.5 metres in length, and two tigers, the biggest being about 4.2 [metres long].

"[It was] a very impressive display as they casually moved in and just took large bites out of the dead sperm whale."

Mr Vercoe said it was a rewarding experience.

"It was incredible to find that happening just five or 10 kilometres from Coffs Harbour. Unbelievable. It was a really exciting time," he said.

The whale carcass later washed up on shore.

"She had obviously drifted over quite a distance," Mr Vercoe said.

"Normally their habitat's about 40 kilometres off-shore.

"So, to find her only 500 metres off the beach, she'd obviously been adrift for quite some time.

"She'd been dead for at least a week I imagine for her state."

The footage emerged after two surfers were attacked by sharks on the NSW north coast this week.

One of the surfers, a 32-year-old man, had his legs mauled by what was believed to be a great white at Ballina yesterday and is reportedly in a critical condition in Gold Coast Hospital.

A 52-year-old man was knocked off his surfboard by a shark at Lennox Head this morning.

Paramedics said the man was not injured in the incident.