Zone 5 is located in the lower Tamar River estuary and extends from George Town to Bass Strait, 3.5km past the mouth of the estuary. This zone is where the estuary meets the sea and as a result the salinity here is predominately marine except in times of large floods moving down the estuary. The salinity in Zone 5 ranges from 29.5- 35.4 ppt with a sample depth range of 14-40 m. This area within the estuary has the largest variety of benthic habitats, comprising a mixture of sand, seagrass, sponge garden, cobble and reef with sandy rocky shores and sandy beaches closer to the ocean (Lucieer et al., 2009). This zone is the entry to the Tamar River estuary and is a shipping route to the industry ports of Bell Bay and Longreach. As the area is the main shipping route from Bass Strait to the ports in the estuary, Zone 5 also has a number of invasive species that have been introduced via shipping activities such as the Pacific oyster, Asian bag mussel and the European green crab. Zone 5 is also used for recreational activities such as swimming, boating, fishing and scuba diving.
Due to the distance from the upper estuary and freshwater inflows, surface and bottom waters have similar salinity values, although a slight “freshening” of the surface waters is occasionally observed.