No introduced marine pests, also known as invasive marine species, were detected in a dry season survey in August 1998 in Darwin Harbour, Northern Territory, Australia. A postwet-season survey in March 1999 found extensive populations of the false mussel Mytilopsis sallei (Récluz, 1849) in the Cullen Bay Marina. An environmental emergency was declared and M. sallei were eliminated. The present paper examines possible reasons for the failure of M. sallei to establish outside Cullen Bay. Mytilopsis sallei is a fresh/brackish water species that naturally lives in low salinities. It is hypothesized that the osmotic shock of rapid movement from low salinity water in Cullen Bay to much higher salinities outside the marina killed the veligers when they were discharged through the entrance lock when it was opened to allow vessel movements. If it entered the harbour, M. sallei could possibly have established in low salinity water in the arms leading into the harbour and possibly have spread in northern Australia.