A "mystery" marlin that washed ashore at a Napier beach gave some early morning photographers a fright.

The largely intact marlin corpse was first spotted near the Awatoto treatment plant outfall area on Wednesday morning, well away from its usual deep-water habitat.

National Aquarium of New Zealand general manager Rachel Haydon was unable to determine the exact species of the fish based on the photo, but said she was surprised to see the marlin beached.

"There are a number of species known to frequent New Zealand waters seasonally – Swordfish, Striped Marlin, Black Marlin and Blue Marlin," she said.

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"It is unusual to see one washed up a beach and we have not heard of any others washing up nearby recently."

Haydon said ill health causing death, ingestion of something or lack of food supply could have been the reason behind the marlin coming ashore.

Fishing of marlin species is usually more successful in summer months with more abundant numbers.

"Striped marlin, that can grow up to and in excess of 200kg, usually arrive in warmer waters during December or January and can range as far south as Gisborne and New Plymouth," Haydon said.

"Blue marlin, with an average size of 200kg, can make seasonal migrations into the temperate New Zealand waters for feeding in spring and summer too – so why it has washed up in winter is more of a mystery."

The public are encouraged to contact the Department of Conservation on 0800 362 468 for any sick, injured or dead wildlife spotted across the region.