Dozens of rescue crews are scouring the NSW north coast for a shark, believed to be a great white, that fatally attacked a surfer on Monday morning.

Tadashi Nakahara, 41, a Japanese national who lived in the area and was popular in the local surfing community, died at Shelly Beach, a well-known tourist spot in Ballina, just before 10am, police said.

Surfers who were in the water at the time said they saw the shadow of the shark and estimated it to have been between 3.5 and four metres long, the Northern Star reported.

Two close-by surfers pulled Mr Nakahara from the water and administered first aid but he died on the sand. Two others paddled out to help apply an improvised tourniquet to the man's wounds.

"They did their best and unfortunately they couldn't stop the blood loss," Detective Inspector Cameron Lindsay, from Richmond police, said.

It is believed the shark bit off both of Mr Nakahara's legs, but police have declined to confirm these reports.

"We believe it was a very large shark," Detective Inspector Lindsay said.

Ballina Shire mayor David Wright said rescuers believed it was a great white shark.

"For a shark to take the board and the person sitting on it, it's got to be very big," he said.

Police are waiting for scientific advice about the species of shark, which will help them determine how much risk it poses to swimmers and whether it was the same shark involved in another, non-fatal attack 25 kilometres away on Sunday.

Mr Nakahara was sitting on his surfboard when the shark came up beneath him, taking off both of his legs and pulling him off his surfboard, Detective Inspector Lindsay said.

Police said they have been in contact with the Japanese consulate.

Mr Nakahara is believed to have worked at a Ballina hotel and was well known in the local surfing community.

Police are at the scene and lifeguards are searching for the shark in jet-skis, rescue boats and a helicopter, a Surf Life Saving NSW spokesman said.

All beaches along the 15-kilometre stretch from Lennox Head to South Ballina have been closed at the direction of police.

"He was a really nice guy, he loved his surfing," Richard, the manager of a surf shop, said of Mr Nakahara.

"He always had friends coming out from Japan and he'd always take them surfing."

#BREAKING Fatal shark attack at Shelly beach in Ballina. It's thought the victim is a Japanese tourist. pic.twitter.com/pX8vhghEoP — Ashleigh (@ASaunders_) February 9, 2015

Richard said the frequency of shark attacks in the area was "getting crazy".

"Every week we get someone knocked off their board or nipped, it's getting ridiculous," he said.

A crime scene has been established at Shelly Beach and a report will be prepared for the coroner. Police have seized video taken at the beach which is believed to have captured the aftermath of the attack.

"We are now concentrating on searching the area and seeing if we can locate that shark and any remains," Detective Inspector Lindsay said. "We will seek scientific advice [on what kind of shark it was]

Natasha Loosemore, a worker at the nearby Dunes accommodation facility, said she heard "a lot of sirens" and saw everybody out of the water.

"It's a surfing beach but it's also a family beach as well, which is pretty scary," she said.

Cr Wright said it was likely the shark at Shelly Beach was the same one that bit a man at Seven Mile Beach near Byron Bay, 25 kilometres away from Shelly Beach, on Sunday morning.

The surfer, believed to be from Byron Bay, suffered a cut to the right side of his back and puncture marks to his buttocks.

 

There have been a spate of shark sightings and attacks along the NSW coast this summer, forcing some beaches to close for weeks.

Australian Ironman and Olympian Ky Hurst took to social media to warn off using the attacks as a basis for a shark cull.

"It's tragic there have been [two] attacks in the last day in NSW," he wrote. "But understand that it's [sharks'] back yard not ours.

"If people don't like the thought of sharks or the possibility of being attacked then it's very simple, stay out of the ocean.

A 1.8-metre shark was sighted off Newcastle's Bar Beach on Friday and a bodysurfer was bitten on the foot by a small shark at nearby Merewether Beach.

Paul Wilcox, 50, died after being mauled by a great white shark at Clarkes Beach in Byron Bay in September last year.

However, a spokesman for the Department of Primary Industries said on Monday that there was no evidence to suggest increasing populations of potentially dangerous sharks along the NSW coast.

A shark biologist from the department is travelling to Ballina and will study the attack pattern and assist in identification of the shark.

with James Robertson