A leatherback sea turtle has washed up in Pigeon Bay, in the Banks Peninsula, stunning visitors to the area.

The male leatherback reportedly weighed approximately 300kg and is believed to have already been deceased when it washed up on the beach.

A photo posted to Facebook on Tuesday shows the turtle on the back of a truck.

"Definitely something you won't see every day," Dave Hyatt said in his post with the photo.

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"Unfortunately deceased but incredible all the same," he added.

The photo was shared widely across social media.

"What an incredible sight of the most endangered marine turtle subpopulation of the Pacific. Sad to see that this leatherback turtle was found deceased. Known as a leatherback as it is covered by skin and bony flesh, unlike most turtles with a bony shell," Banks Peninsula Geopark said in its post.

The endangered animal was taken to the Department of Conservation officials and there are reports it will be sent to Te Papa.