A teenager has sustained "significant injuries" after he was attacked by a shark while spearfishing off the coast of Arnhem Land.

Key points: The teenager had been spearfishing with friends, who stemmed the bleeding before paramedics arrived

The teenager had been spearfishing with friends, who stemmed the bleeding before paramedics arrived He is being transferred to the Royal Darwin Hospital after initial assessments in Nhulunbuy

He is being transferred to the Royal Darwin Hospital after initial assessments in Nhulunbuy St John Ambulance representative says shark attacks in the Northern Territory are rare

The 17-year-old was bitten by what authorities believe to be a reef shark off the coast of Nhulunbuy, around 970 kilometres east of Darwin, on Sunday.

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

Emergency services were notified of the incident at around 10:00am ACST.

The teenager was bitten by what authorities believe to be a reef shark ( Supplied: Mathew Vickers )

Craig Garraway from St John Ambulance said the 17-year-old was taken to the Gove District Hospital where his injuries were assessed, before being airlifted to the Royal Darwin Hospital.

"Due to the significant injuries to his arm, they decided he would be best flown to Darwin for further treatment," Mr Garraway said.

"Obviously there's quite a large amount of bleeding that's occurred.

"I believe the people on the boat have done an amazing job to stem the bleeding and to give him first aid treatment prior to the arrival of paramedics."

The 17-year-old arrived in Darwin in a stable condition on Sunday afternoon.

Shark attacks 'very rare' in the Northern Territory

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

"The click of the trigger was sending them in a frenzy and charging," one user wrote.

"[The sharks] have been a bit savage up there lately," wrote another.

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

However, Mr Garraway said shark attacks in the Northern Territory were rare.

"I've been around a long time and I'll be honest, I can't remember a shark attack," he said.

"I did time over at Nhulunbuy and I've been involved in Nhulunbuy for a long time and I can't recall one over there."