A large black coral believed to be centuries old is one of the largest ever found in the waters of New Zealand, researchers say.

The coral, measured at 4m high and 4.5m wide, was found in the waters of Fiordland.

It was likely to be 300-400 years old.

Victoria University marine biologist Professor James Bell said the coral was "absolutely huge".

"It's by far the largest black coral I've seen in my 25 years as a marine biologist. Most black corals we come across when we're diving are small, with the bigger ones usually less than two to three metres tall, so finding this one was really cool."

The discovery was considered significant as large corals provided vital breeding stock for the species, which were slow to grow.

"Pinpointing where large corals occur means we can better protect them by letting people know where not to anchor their boats or drop pots," Bell said.

Despite its name, the black coral was white in colour and only its skeleton was black.

Department of Conversation (DOC) staff were also on the dive when the coral was found.

Senior biodiversity ranger Richard Kinsey said seeing the large coral looming out of the darkness was "pretty special".

"I've been a marine ranger in Fiordland for nearly 20 years and it's rare to see a coral so big. It's easily the largest one I can remember seeing."

Victoria University researchers were working alongside DOC and the Fiordland Marine Guardians to study and map the distribution of protected coral species in the fiords.

"We'd love to receive reports from anyone who knows of particularly large black corals that are greater than four metres so we can map their distribution and find out how common such large coral colonies are throughout Fiordland," Bell said.

Under the Wildlife Act the coral was listed as a protected species, meaning it was illegal to deliberately collect or cause damage to it.

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