A teenager who was "mobbed" by sharks while spearfishing off the coast of Nhulunbuy was dragged to safety by a group of quick-thinking friends who stemmed the bleeding for a harrowing two hours until emergency services arrived.

Key points: Sean Whitcombe underwent emergency surgery after being attacked by a shark

Sean Whitcombe underwent emergency surgery after being attacked by a shark Fellow diver Joseph Smith said he pulled the teenager from the water after seeing a "pool of blood"

Fellow diver Joseph Smith said he pulled the teenager from the water after seeing a "pool of blood" An Aboriginal trainee nurse spearfishing with the group saved the day, Mr Smith said

Seventeen-year-old Sean Whitcombe sustained significant injuries and remains in a critical condition in the Royal Darwin Hospital after he was bitten by a three-metre reef shark near Bromby Island, around 970 kilometres east of Darwin, on Sunday.

Careflight said the teenager told a staff member he had been "mobbed" by multiple sharks.

He had been spearfishing from a boat around 30 kilometres offshore when he was bitten on the arm and leg.

Joseph Smith was diving with Mr Whitcombe when he saw a "pool of blood" around 60 metres from the boat.

"I jumped on the tinny and raced over to him and just saw all the blood," he said.

"He'd speared a mackerel and then they took the mackerel off his spear rod … then the sharks turned on him.

The 17-year-old suffered arm and leg injuries in the attack. ( Facebook: Sean Whitcombe )

"He said he saw about seven [sharks], but it was that quick that he didn't see the shark who bit him."

Mr Smith, who was with his eight-year-old son at the time, said he ripped the teenager out of the water, leaving his diving gear "floating in the ocean somewhere".

"As soon as I got him onto the tinny, I raced to the big boat while driving and applying first aid," he said.

"It was confronting; major veins were severed … straight away I ripped my shirt off and started wrapping [it] around him.

"Once he was on the big boat, that's when I did sort of start to panic."

'Joe, am I going to die?'

"Chiko", an Aboriginal trainee nurse who had been spearfishing with the group, administered first aid while Mr Smith activated an emergency radio beacon (EPIRB).

But emergency services "got the wrong coordinates", Mr Smith said, and did not find their boat for two hours.

Mr Smith (pictured) said the situation would have been worse if an Aboriginal trainee nurse was not on board. ( Facebook: Joseph Smith )

It meant the group had to keep Mr Whitcombe calm and stem the bleeding until police arrived.

"We just spoke to him about happy days … I just tried to keep him in a good headspace," Mr Smith said.

"He said, 'Joe, am I going to die? Is me [sic] arm going to be chopped off?'

"If we hadn't had Chiko with us, it would have been a lot worse … it was unbelievable."

The teenager has since undergone emergency surgery at the Royal Darwin Hospital.

St John Operations Manager Craig Garraway said emergency services had worked tirelessly to save his arm.

"Even with the best first aid that was given, he had lost quite a lot of blood along the way," he said.

"So those injuries were severe and I think the idea of surgery was to try and at least repair that arm."

On social media, users of a local spearfishing group warned that sharks off the Arnhem Land coast had been "the worst" they had seen in years.

In September, a 17-year-old boy sustained knee injuries after he was attacked by a bull shark off Groote Eylandt, south of Nhulunbuy.