On the evening of 5 January 1975 the bulk carrier Lake Illawarra, inbound to the Risdon zinc works with a cargo of zinc concentrate, struck pylons 18 and 19 of the Tasman Bridge. A 127 m section of the bridge collapsed onto the ship, which sank within minutes. Seven crew died, and four cars drove off the broken span into the river, killing five motorists; two other cars stopped with their wheels over the edge.

The disaster cut Hobart in two for almost three years and led to compulsory pilotage on the river. The ship – 139.8 m long, 7,274 GRT and 10,380 DWT, built by BHP at Whyalla and delivered in 1958 – lies in 35 m just south of the bridge, with concrete bridge sections on and around her. Her bunker oil was pumped out and the bow later removed, and the wreck is deep enough to pose no hazard to shipping.

The Tasmanian Scuba Diving Club describes light penetrating to about 26 m, roughly deck level, with features including the bridge structure rising about 15 m, funnels about 10 m high, radar domes, hoists and portholes, and the hull serving as an artificial reef. The Marine Life Network guide calls it the spookiest dive in Tasmanian waters: jewel anemones share the structure with thick river silt, visibility is often near zero, penetration is dangerous, and the bridge is directly overhead. Debris and recording instruments around the bow are off-limits.

The site is also a place of loss: twelve people died here. This is an expert dive done only with TasPorts permission, strong wreck and low-visibility skills and full surface support. It is listed for its historical significance rather than as an accessible recreational dive.

Boat access only, and only with permission. The wreck lies directly beneath the Tasman Bridge off Rosny Point, in the main navigation channel of the Derwent. The Marine Life Network guide states that TasPorts permission is required and that police boats patrol the area.

Clubs that have dived it launched from ramps such as Lindisfarne, Bellerive or Kangaroo Bay, a few minutes away. Never attempt the dive without written permission, a dedicated surface-support boat and coordination with port traffic.

The Derwent estuary is sheltered from ocean swell, but strong northerly or southerly winds blow along its length and raise a short, steep chop; the Marine Life Network notes the region can become very rough in gales, even in the river. Visibility is the limiting factor: it is usually low (1-5 m) and worst after rain, when the Derwent's tannin-stained, silty floodwater spreads through the estuary. Dive after a dry spell, on a high or flood tide that brings in clearer marine water.

There is a noticeable but generally mild tidal flow. Diving is possible year-round; winter water is cold (9-11 °C). The Marine Life Network recommends a noon summer flood tide after a dry period for the best light and visibility on the wreck.

With permission and surface support, descend a shot to the upper superstructure at about 24 m, examine the bridge and funnel area without penetrating, and ascend on the shot with a planned safety or decompression stop. Stay well clear of the bow debris and monitoring instruments.

Low-visibility wreck skills, reel, redundant torches and gas; plan conservatively for 30-35 m in near-dark water. Penetration not recommended. Surface support boat mandatory.

No commercial operator. Occasional authorised dives by the Tasmanian Scuba Diving Club and other experienced groups with TasPorts permission.