The Lake Illawarra was a steel, single screw, steam turbine motor vessel bulk carrier. The Lake Illawarra sank after running into and demolishing part of the Tasman Bridge that crossed the Derwent River at Hobart on 5 January 1975. The vessel was en route from Port Pirie in South Australia with a cargo of zinc.

When approaching the bridge, the captain reduced speed, but was finding it difficult to align the ship to the central navigation span of the bridge. Despite several attempts to change the course, the ship was difficult to manage, in part due to the lack of speed. The master finally called out full speed astern in a last attempt to change the course of the ship, when the ship veered into the pile capping of Pier 18 and then Pier 19, bringing the three unsupported spans of the bridge crashing into vessel's hull. The ship sank within minutes in deep water.

Seven crew members on the ship died and four cars ran over the gap into the Derwent River. Five occupants died; others managed to escape from the two vehicles which teetered on the edge of the gap. Police and emergency services were called into action; within an hour, a ferry was traversing the river.

The collapse effectively divided Hobart in two. The next day there were three private ferries and one government-run ferry in operation. Eastern shore residents were severely hindered by the collapse as most hospitals, schools, businesses and government offices were located on the west shore.

A temporary bridge (the Bailey bridge) with one lane in each direction was built and opened to traffic on 16 December 1975. The federal government provided funding to rebuild the Tasman bridge at a cost of $44 million and it re-opened 8 October 1977.