A CLOSE-KNIT scuba diving community is in mourning after the death of one of its members during a "technical dive" off Sydney's northern beaches.

The 41-year-old man was pulled unconscious from the water at 8.30am (AEDT) on Saturday after getting into difficulty at a depth of about 60 metres at a wreck off Barrenjoey Headland.

He was put on board a Broken Bay Water Police vessel, where he was treated by a doctor and paramedic who were lowered from a rescue helicopter.

However, the man could not be revived and was pronounced dead at the scene.

Sergeant Malcolm Jeffs, from the Police Diving Unit, says the man was with a group of seven other technical divers, who were devastated by the accident.

"The technical diving community is fairly close. I'm not sure how well they knew the person, but it's a very upsetting situation for all those involved," Sergeant Jeffs told reporters.

He said the group had been offered counselling services and were still being interviewed by police.

Technical diving uses mixed gas combinations, which vary according to the depth of descent.

Sergeant Jeffs said the man's diving buddy conveyed him to the surface immediately after noticing he was in difficulty.

"Treatment appears to have been given as fast as possible. There's no indication that there was any delay in treatment or response to his situation."

Sergeant Jeffs said the police investigation was still in its infancy so it was not immediately clear what had caused the accident.

"We don't know whether it was equipment failure or some condition with him, so that's all part of the investigation that will be prepared and passed onto the coroner."

The man's family is yet to be informed of the tragedy, he said.

One of the divers that tried to help the man developed symptoms of decompression illness and was flown to the Prince of Wales Hospital, where he remains in a stable condition.

The man's diving gear has been seized for scientific examination.

Investigations are continuing and a report will be prepared for the coroner.