The collision occurred at 9:27 p.m. (Australian Eastern Summer Time UTC+11) on Sunday 5 January 1975. The bulk carrier "Lake Illawarra", carrying 10,000 tonnes of zinc ore concentrate, was heading up the Derwent River to offload its cargo to EZ Industries' Risdon Zinc Works, upstream from Hobart and about 3 km from the bridge. The 1,025 m long main viaduct of the bridge was composed of a central main navigation span, two flanking secondary navigation spans, and 19 approach spans. The ship was off course as it neared the bridge, partly due to the strong tidal current but also because of inattention by the ship's master, Captain Boleslaw Pelc. Initially approaching the bridge at eight knots, Pelc slowed the ship to a 'safe' speed. Although the "Lake Illawarra" was capable of passing through the bridge's central navigation span, the captain attempted to pass through one of the eastern spans.