Swimmers at two of coastal New South Wales' most popular tourist spots are being urged to tread carefully, amid fears of razor clams in the shallows.

The fan-shaped clams, which are also known as razorfish, can grow up to 50 centimetres long. They bury themselves in sand, leaving sharp, upward-facing shell edges exposed.

They are common in Lake Macquarie, north of Sydney, and Jimmy's Beach near Hawks Nest, north of Newcastle.

Warning: this story contains a graphic image that some readers may find distressing.

They are also found in shallow coastal waters in most Australian states and there has been a push by some residents to get rid of them.

Shark fears prompted by bloodied, severed tendon

Former Jimmy's Beach local Jennie Perkins said she was worried that razor clam numbers had spiked off the coast at Hawks Nest over the past decade.

The keen windsurfer said she would never forget a razor-sharp clam severing her tendon a few years ago.

"It was low tide and so I just jumped off my board into the sand and straight on to the razor shell," she said.

"I felt something and it wasn't until I lifted my foot up and saw the big flap of skin and blood everywhere.

"I thought, 'Holy hell, there are sharks around here' and I waved some guys down in a boat.

There are fears that razor clams are on the rise at several NSW tourist hotspots, including Jimmy's Beach north of Newcastle. ( Supplied: Great Lakes Shire Council )

"It was an ordeal because I knew I had to get back to the beach as soon as I could.

"It was an adventure that I won't forget."

Ms Perkins said she required 20 stitches and spent several days in hospital.

She said it was as though her foot had been sliced open with a can opener.

"It just sliced it straight open, and there were no jagged edges or anything.

"As the surgeon said, 'Oh it is very easy to sew back together again'."

Tea Gardens beachgoer Jennifer Winney said she was also worried about a rise in razor clams at Jimmy's Beach.

"I've noticed an increase in numbers in the last three years," she said.

"I can't ever recall seeing them before at Jimmy's Beach but they are there now in significant numbers."

Ms Winney said anyone swimming in the area should wear shoes.

"I have friends who are paramedics here and they are often called for lacerations from these nasty shells," she said.

"They just bury themselves point down into the sea bed and there is an exposed razor edge."

Razor clams have caused concern in Lake Macquarie for several years. ( Supplied: Steve Dunn )

Clam fears in newly opened coastal baths

Razor clams have long been a problem in Lake Macquarie as well.

Locals and the lake's recreational users have repeatedly called for the removal of the clams from popular swimming areas due to the hazard they pose.

The latest fears have been raised about the newly opened Belmont coastal baths — a protected swimming enclosure in the lake surrounded by mesh on three sides.

A parent issues a warning on social media after a razor clam sliced open a child's foot. ( Supplied: Nardi Janahli Simon )

A photo of a child's severed foot on social media this week has reignited debate about the clams, with the photograph's caption warning parents to be on the lookout for clams.

"To the people that think it is safe to take your kids down to Belmont Baths because they say it's all fixed up, this is what happens," Nardi Janahli Simon said.

Lake Macquarie Council manages the baths and its recreational and land planning coordinator Stephen Prince said he was aware of the concerns.

"Certainly prior to us opening the facility we had a marine ecologist do a snorkel survey for us of the entire swimming enclosure to make sure it was free of debris," Mr Prince said.

"We had a dozen clams taken away from the enclosure, so it is very unfortunate that there was a razor clam that wasn't able to be found."

Mr Prince said each year popular swimming areas were inspected to find the clams and remove them.

He said the Belmont Baths would now be re-inspected to make sure any remaining clams were taken away.

"Right now we are arranging for another inspection next week and we want to try to find the razor clam that the person did cut themselves on.

"We will go through and sweep the entire area."

Lake Macquarie Council says it will investigate the latest concerns about razor clams. ( ABC )

Academics caution against removing clams

In the past decade, Lake Macquarie Council has engaged university academics to study the lake's clam population, in response to residents' concerns that numbers had spiked since commercial fishing activities were curbed.

The study, headed up by Peter Macreadie from the University of Technology in Sydney, found the clams should stay put.

"We do not recommend the removal of razor clams from the lake," Dr Macreadie found.

"That said, if razor clams are to be removed, then we suggest that they be transplanted to strategic locations around the lake so that populations can be maintained," he said.

"Furthermore, we suggest that genetic analyses of razor clam populations within the lake may help to identify … populations, which would help to maintain the conservation of the species while also avoiding human conflicts."