By Kim Griggs in Wellington, New Zealand

What is thought to be the biggest octopus ever found has been caught in waters off New Zealand.

Then I had a freezer clean-out and I had no idea what it was

Dr Steve O'Shea, NIWA

Marine biologist Dr Steve O'Shea estimates the specimen, which was damaged when fished up, would have measured four metres in length and weighed 75 kilograms.

"That's a conservative estimate," Dr O'Shea told BBC News Online. "It is an absolutely massive octopus."

The incomplete specimen has a mantle length (the standard measure of length in octopus and squid) of 0.69 metres, a total length of 2.9 metres and a weight of 61 kg.

Not a squid

Octopus (Haliphron) had previously been thought to reach a mantle length of only 0.4 metres and a total length of 2 metres.

Dr O'Shea does not think this animal is native to New Zealand waters

"Nothing remotely comparable to the size of the New Zealand specimen has ever been described before," Dr O'Shea said.

The octopus was caught last October in 920 metres of water south east of the Chatham Islands, by the research ship of New Zealand's National Institute of Water and Atmospheric Research (NIWA).

At first, Dr O'Shea paid little attention to the red gelatinous specimen, thinking it was just another example of his research specialty, the giant squid.

Freezer clear-out

"I have a freezer full of squid. And I looked at this and I just thought, 'Heavens, it's a pretty beat up sort of squid'. And I wasn't in any hurry to defrost it. Then I had a freezer clean-out and I had no idea what it was."

He has provisionally identified the sub-mature female as being Haliphron atlanticus. Adding to the mystery, this particular species has never been caught before in the South Pacific.

There are some records from around Japan, Papua-New Guinea and from the Atlantic. "The New Zealand form that we have is more similar to a species which was recorded off Japan in 1902 than it is to the Atlantic species.

"So although I call it Haliphron atlanticus, that's a very provisional identification."

Splendid sight

Dr O'Shea is also puzzled by the fact the New Zealand research institute has never seen juveniles of this species in New Zealand waters.

This is despite the fact that the area where the octopus was found is extensively trawled by commercial fishing vessels and unusual specimens are routinely passed to NIWA.

"I don't believe that this animal is residing in New Zealand at all. It could have been something that's migrated in from spectacular depth.

"Not only is it not residing in New Zealand waters, I don't believe we get the full life history of the species in New Zealand," he said.

The undamaged octopus would have been a splendid sight: all the arms would have been connected by a thick web.

"It would have looked like a huge jellyfish or a great big thick umbrella," Dr O'Shea said.

Pictures courtesy of the New Zealand National Institute of Water and Atmospheric Research