In 1826 the Land Commissioners highlighted the difficulties most travellers experienced in crossing the Derwent, with the main north-south route from Hobart to Launceston following the river to Roseneath (now Austins Ferry) where James Austin conveyed them by punt across the river. The Land Commission recommended the construction of a new crossing, offering a safe and speedy transit over the Derwent. Their preferred location was at Black Snake (now Granton) where "an excellent quarry and the advantage of a sand bank extending half a mile in length to the Channel, which is here only three hundred and fifty yards across" were highlighted.
The settlement on the other side of the Derwent was originally known as Green Point until it became known as Bridgewater simply because it was the bridge (actually a causeway) crossing a shallow section of the waters of the Derwent River.
The first ferry service across the Derwent was established in 1816 by James Austin and his cousin John Earle. It remained vital to travellers journeying from Hobart Town to Launceston until the completion of the causeway. By 1820 Austin and Earle were using a punt capable of transporting 30 cattle, 200 sheep or two carts and 16 oxen. In 1829 construction began on a causeway across the Derwent River. It was 1.3 km long and was built by a gang of 200 convicts using nothing but wheelbarrows, shovels and picks. By the time it was completed the convicts had shifted 2 million tonnes of sand, soil, stones and clay. Defined as secondary punishment for serious reoffenders, if the convicts were considered to have not done a full day’s work they were placed in solitary confinement in a cell which was only 2 m high and 50 cm square. The causeway was completed in 1836. It did not traverse the river and so a ferry plied the deepest section of the Derwent River until a bridge was completed in 1849.
The Old Watch House at the southern end of the Bridgewater causeway was built by convicts in 1838. It was constructed to house the soldiers who supervised the convicts. Behind the building is the quarry which was used to provide the fill for the causeway. Considerable problems were encountered establishing a firm base to the construction works because of the river silt and the clay base. The work was referred to as the "Bridgewater Folly", where men tried for years to fill up a soft mud hole. The Old Watch House contains the notorious solitary cell which was used to punish convicts building the causeway.