Thousands of by-the-wind sailor velella washed ashore at Cable Bay, north of Nelson on Saturday.

Residents of Cable Bay have been checking their nature books and google to identify a rare visitor to their shores.

"We're all a bit more educated out here now," said Barbara Stuart, who took this photo of a swarm of by-the-wind sailor Velella Velella that washed ashore on the beach on Saturday morning.

"We had never seen anything like this before, and my husband has lived here all his life."

The by-the-wind-sailor is not a jellyfish, but hydrozoans. Most Velella are less than 7cm long and usually deep blue or purplish in colour.

Their most obvious feature is a small stiff sail that catches the wind and propels them over the surface of the sea.

Under certain wind conditions, they may be stranded by the thousand on beaches. While the sting is not deadly to people, it can cause welts on the skin.