It might appear to good to be true, however, an Auckland marine scientist says a koru forming in the Hauraki Gulf is completely plausible.

Todd O'Hara was flying in a small aircraft to Waiheke Island on Saturday when out of the corner of his eye he spotted a strange formation on the water.

The large spiral-like koru spanned across the coastline of Ponui Island and looked as though someone had pulled the plug out of the Gulf.

"I've done quite a bit of flying around the Auckland area and parts of the country but I've never seen anything like this," O'Hara told the Herald.

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"We were just flying past and I saw it out of the corner of my eye and it was unusual looking ... to see something like that was a bit different."

University of Auckland marine scientist Dr Andrew Jeffs said the formation was "a beauty" and speculated a type of phytoplankton was behind the koru.

Photograph of a suspected phytoplankton bloom swirling near Ponui Island. Photo / Todd O'Hara
Photograph of a suspected phytoplankton bloom swirling near Ponui Island. Photo / Todd O'Hara

"There's a group of algae called coccolithophores which have that characteristic white colour and they're seen blooming in that kind of way," he said.

Jeffs said it was impossible to tell for certain if it was the specific algae without a sample but he said the colour of the bloom fits the bill.

Coccolithophores are single-celled phytoplankton which live in large numbers throughout the upper layers of the ocean.

Similar blooms do occur in the ocean off the New Zealand coast Jeffs said, who has seen satellite images of bigger versions.

"When they get the right conditions they multiple very very quickly," he said.

"Those ones are particularly noticeable because of their colouration but that sort of thing is going on all the time in the ocean."

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In this case, Jeffs said it appeared there was a patch of perfect conditions which has allowed the phytoplankton to bloom before a current swirled them around.