Oysters in the Mullet Creek, the Hawkesbury's tributary, have been struck down with the Pacific oyster mortality syndrome, affecting about 20 percent of the local industry.
According to SMH, the tributary is "the local industry's nursery for juvenile stock", and oysters which were perfectly healthy on Monday – millions of them – have been wiped out overnight.
Many of the area's 15 growers are now sceptical that they'll have any produce remaining by the end of the year.
Oyster farmer Rob Moxham told SMH, "In 24 hours it wiped out probably about 90 percent of the oysters over a 50-hectare area … It's just so devastating."
The outbreak causes no risk to humans, but has affected about 20 percent of the local industry's 245 hectares.
A quarantine in now in place to prevent its spread, and Ian Lyall, the Department of Primary Industries will be meeting with farmers next Tuesday to discuss assistance.
It's thought that the recent heatwave could have reduced the oysters' immunity to the virus, which could also have spread naturally in ocean currents or via boat hulls or commercial ships' ballasts.