A huge mass blankets an 18m by 8m area of the Blaketown aerodrome car park, with more scattered south along the high tide mark.
By last evening, they were giving off a strong smell as they lay in the sun.
DOC marine expert Don Neale said by-the-wind sailors were related to jellyfish and 'bluebottles', which also often washed up.
"They're quite often seen washed up on West Coast beaches and when there's one there's usually dozens, but I've never seen them drifted up this much (in their thousands) in one place before," Mr Neale said.
"As the sea starts to warm up in the spring, all the floating plankton starts to bloom and it's not unusual to get a lot more things like this starting to wash up, as well as 'red tides' of plankton out at sea."
Just over a week ago, a large number washed up at Cable Bay, Nelson, and a few days later also at Farewell Spit.
They get their name from the small erect 'sail' on their backs, which allows propulsion by the wind. They are not considered dangerous and are part of the Cnidaria family.
