An unusual cloud formation which appeared to be out to sea off the Hawke's Bay coast on Monday may have been from a burnoff in the area of Mahia Peninsula.

Viewing the phenomena while having breakfast at Clifton cafe, Rachael Bailey said it looked "like a volcano erupting out at sea ... clear blue skies then white smoke/steam pouring out the ocean."

"We wondered if it could be a ship on fire but thought the smoke would be black," she said.

"It wasn't moving like a cloud. Staying in the same spot only getting bigger."

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The cloud was also reported from vantage points at Westshore, appearing to be offshore and spreading to the south, consistent with the northerly breezes during the reporting of the cloud from late-morning to early-afternoon.

A New Zealand Fire and Emergency spokesman was unable to offer any other explanation, saying no related calls had been received by the service although it was aware of a burnoff in northern Hawke's Bay.

Police also had received no calls, and MetService meteorologist April Clark was unable to determine the source from a satellite image.