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A poisonous sea creature with toxin potent enough to kill someone in 20 minutes has been spotted in the waters off Northland.

Dive instructor Alan Morrison photographed the carnivorous cone shell while diving at Deep Water Cove in the Bay of Islands.

The marine snails have previously been seen in the Parengarenga and Whangaroa Harbours.

The cone shells' potentially lethal venom can be delivered through gloves or a wetsuit, even when handled carefully.

"I was about 300m from the frigate Canterbury when I saw the 6cm-long cone shell. It was about 10m down the bank of an island and stuck to the side of a rock."

Mr Morrison said he was running out of room on his camera's memory card but managed to take a few pictures.

"Luckily, I knew what a cone shell looked like. I was aware of their reputation and knew it wouldn't be a good idea to pick it up."

Once back on land he sent his pictures to marine expert Wade Doak, of Ngunguru, who confirmed his suspicions.

Mr Doak said cone shells were either herbivores or carnivores.

The herbivores attached themselves to reef crests where they ate algae.

"Carnivorous cone shells, like the one seen at Deep Water Cove, have lighter shells and they fire their weapon at small fish like a whaling harpoon. It's linked to the animal by a translucent tube down which a fatal venom is injected."

The cone shell retracts its tube and draws the prey into its mouth.

After the prey is eaten, the marine mollusc regurgitates any indigestible material such as spines and scales, and the then disposable harpoon.

Mr Doak said cone shells were capable of killing fish bigger than themselves.

Marine author Tony Enderby said the variety of cone shell found at Deep Water Cove - Conus lischkeanus kermadecensis - had been found at the Kermadec Islands and in Parengarenga and Whangaroa Harbours, but nobody in New Zealand had died after contact with them.

He suspected it was the only variety of cone shell found in shallow waters - from the low tide mark up to 30m deep - around mainland New Zealand.

Most fatalities occurred in the tropics after people picked up cone shells while walking on the sea shore, Mr Enderby said.

"If you were to stand on one they wouldn't have time to react, but if you pick them up and hold them in your hand, the animal senses something that might be food and stabs it."

Mr Doak's advice to divers was: "Don't interfere. Take pictures but leave them alone.

"Let them be part of the wonderful diversity that we can enjoy on the undersea cliffs."