Tens of thousands of jellyfish-like creatures have washed up on beaches on New Zealand’s South Island.

The velella velella, which are related to jellyfish and also known as by-the-wind-sailors, blanketed patches of beaches on the island’s west coast, the Greymouth Star reports.

They covered an area of 18 x 4 metres on one beach and soon began rotting in the sunshine, exuding a strong smell.

Conservation officials say it is common for the jelly creatures to wash up in the spring as water temperatures rise, but added they were surprised by the vast number.

“It’s very unusual that you have so many thousands of them washed up in one place,” the country’s Department of Conservation (DOC) said.

“It’s a common species around the west coast and around NZ and they do wash up, usually in tens or hundreds.”