A creature apparently photographed at Swansea, south of Newcastle, has confused and slightly frightened locals since it washed up on social media on Monday, the Newcastle Herald reports.

Ethan Tippa, who posted the photo on Facebook, typified the general response.

"What the f--- is it?" he asked. 

The answer, said marine biologist Julian Pepperell, is that it's a pike eel.

The angle of the photo made it difficult to judge the creature's length, but it seems longer than the species' average maximum of 1.8 metres.

"I think it's definitely a pike eel. The head is very indicative of that species," Dr Pepperell said.

"It's hard from the photo to get an idea of the scale."

The nocturnal pike eel is common in NSW waters, but surprisingly little is known about it.

Dr Pepperell said the species is frequently caught by fishers at night who get "the fight of their lives" when they reel in a powerful, thrashing predator with a nasty bite.

"There are certainly people who are bitten by them in boats," he said.

"They have incredibly strong muscle and their teeth are geared towards inflicting slashing wounds."

An old fishers' adage goes that a tinnie has room for a fisherman or a pike eel, but not both.

The photo of the long, sharp-toothed and clearly dead creature has been shared thousands of times on social media, with many identifying it as a pike eel.

It was probably "relatively old", Dr Pepperell said, and could have died from a net entanglement, been hit by a boat or died of old age.

Some questioned whether the image was Photoshopped, or whether it had really been seen at Swansea.

Dougie Boyd, of the Commercial Fishermen's Co-Operative, thought it was genuine.

"I don't think it's been Photoshopped," Mr Boyd said.

"I've seen some monsters, but none that big."

Pike eels are not poisonous to eat, and are frequently sold in the market of southeast Asia.

Comment is being sought from the Department of Primary Industries.