A smack of jellyfish flooded a Southland fish farm last week, killing thousands of salmon.

Sanford Southland manager Tommy Foggo said the jellyfish killed about 2000 salmon, likely by clogging their gills with mucus, after they were washed up from the sea into the company's salmon farm at Stewart Island last week.

The jellyfish had since floated away, and the deaths were not a major concern especially compared with the more than 200 tonnes killed in 1998.