"This is very unusual," said Mike-Reade Anderson, operations head of the KwaZulu-Natal Sharks Board, as he announced the extension of the bathing ban.
"In my 31 years, this is the first time they have stayed for so long."
Not only are there higher numbers of sharks in the water during the sardine run, the shark nets that usually protect bathers are also removed to avoid trapping dolphins and other marine predators attracted by the shoals.
"You have this whole parade - this menu as you could say - coming up the coastline, of one predator feeding on the next one," Shark Board spokesperson Debbie Hargreaves told RFI.
"So the danger is not just the sardines - it's what's following the sardines and what's predating on the big game fish."
No one will be allowed to swim off Durban's beaches until the sardines move on. The authorities have no idea when that will be, Anderson-Reade said.
Every year between May and July, millions of sardines travel north from the cold southern Atlantic oceans off Cape Point, along the coastlines of the north Eastern Cape and KwaZulu-Natal.
Durban continues to attract tourists despite the cancellation of several swimming and surfing events, as people come to watch - and net - the vast sardine shoals.