The harbour is an expansive, almost landlocked, shallow bay of relatively uniform depth ranging from 3 to 7 metres (9.8 to 23.0 ft),[1] which provides safe anchorage from the Roaring Forties that buffet the western and southern coasts of Tasmania.

The harbour is connected by the narrow 12-kilometre (7.5 mi)-long Bathurst Channel to Port Davey, and then water flows into the Southern Ocean.

Like most estuarine systems in southwest Tasmania, the water is stained a deep red-brown due to tannin leached from buttongrass and adjacent heathland.

As with almost all of the estuarine system including the Bathurst Channel, the seabed falls sharply from the shore before hitting the level floor of the harbour which forms a gently sloping basin of mud and gravel with a maximum depth of 9.1 metres (30 ft) near Dixon Island.

The glacially carved landscape of the region has resulted in the shape of the estuarine system being greatly different to other ria formations around Tasmania such as the Derwent Estuary, instead bearing close resemblance to artificial impoundments like Lake Pedder, 30 kilometres (19 mi) to the north.