A new documentary will show the moment a marine biologist was bitten in the head by a shark in Fiordland.

Jenny Oliver - a fisheries analyst with the Ministry of Primary Industries - was on a research dive investigating an invasive form of seaweed when she was attacked by a 3m sevengill shark.

Oliver's dive team were checking the spread of the seaweed - which is a threat to the sharks' ecosystem - as part of a plan to eradicate it, when she endured the all-too-close encounter.

"I was lucky I was diving in cold water because it meant I had thick dive gear on. That provided protection when the shark got a grip on the back of my head," Oliver said.

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"It would be a different story for someone with minimum protection on their body.

"I felt it bear down on the top of my oxygen tank and I realised it must be trying to bite my gear. It circled me after that, and I pulled my hands in close thinking it might try and get my wrist in its mouth.

"Then it went back and bit [my breathing apparatus] again and then the back of my head before my dive buddy punched it."

Footage of the attack features in Discovery Channel's documentary, Sharks of the Shadowland, set to be broadcast here in the network's Shark Week next month.

The incident happened in December 2012 when Oliver teamed up with one of New Zealand's most experienced shark researchers, Kina Scollay.

Little did they know that a dive aiming to improve the sharks' habitat in Fiordland would lead to the terrifying experience and eventually a Discovery Channel special on the mysterious sevengill species.

Not enough is known about this species of shark to establish whether it is endangered.

Although it is smaller than the notorious great white - which grows to around 3m - the aggressive sevengill shark's jaws are filled with rows of razor-sharp teeth.

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Scollay said sevengills usually have little human contact in remote areas like Fiordland and expedition members found the more the team dived with them, the more aggressive the sharks became.

So much so, he said, "it seemed as if they were learning to hunt us.

"The freaky thing about being in the water with these guys is how stealthy they are," he said.

"You don't know where they're going to come from next.

Jenny Oliver and Kina Scollay tracking sharks. Photo / Discovery Channel
Jenny Oliver and Kina Scollay tracking sharks. Photo / Discovery Channel

"Imagine being on the bottom of the ocean at night. It's pitch black. You're completely cut off from the rest of the world and there is a pack of sharks stalking you."

In 2009 Hastings nurse Greg Sims, 49, had a chunk of his leg ripped out by what was thought to be a sevengill and three Invercargill teenagers were attacked in the surf in 1999.

In that attack the fish bit 13-year-old Jenny McDowell's arm to the bone. It also bit Genna Hayward, 13, and Tim Wild, who suffered six puncture wounds.

Last year, Invercargill doctor James Grant was bitten by what was believed to be a sevengill shark.

The quick-thinking doctor fought off the shark, before stitching up a wound to his leg.

* Sharks of the Shadowland screens on December 10 at 8.30pm on Discovery Channel.