RMS Australia was the biggest ship ever lost at Port Phillip Heads and the most public. She was a P&O Royal Mail steamer, a steel screw liner of about 6,900 gross tons built by Caird & Co at Greenock in 1892, 142 m long, with accommodation for 265 first-class and 140 second-class passengers and the fittings of her era – marble baths, mahogany staircases. At 1.45 on the morning of 20 July 1904, inbound from London at 14.5 knots, she struck Corsair Rock. Her pilot was later found to be suffering from Bright's disease, which had affected his eyesight. Everyone aboard was taken off safely, but the ship could not be moved, and a fire of unexplained origin gutted what remained and burned for five days. Salvors stripped her over the following years, and in 1911 the remains were blasted to clear a passage for fishing boats.

What is left is a debris field roughly 300 m long on the shallow Nepean reef, in 3–8 m of water. The Scuba Doctor listing describes the hull as collapsed by explosives and time, with the boilers, crankshaft, propeller shaft and sections of hull 'lying exposed on the bottom in a rough north to south line'. The steel is covered in kelp and coralline algae rather than the sponge gardens of the walls, and the fish are those of the surf zone – sweep, wrasse, zebrafish – with crayfish under the plates and seals from Point Nepean passing through.

The listing calls the dive 'extremely dangerous and difficult' and best at very low tide, and that is the reality: this is a shallow reef at the throat of The Rip, breaking in any swell, with the ebb setting out to sea over it and the flood forming whirlpools around Corsair Rock 180 m away. It is dived rarely, by experienced club divers on the flattest days at ebb slack, and is of interest mainly to wreck historians. A protected historic shipwreck; nothing may be removed.

Boat access only. Charter boats leave from Portsea Pier (Dive Victoria Group, Bayplay) and Queenscliff Harbour (Dive Victoria Group, Red Boats); run time is about 12 minutes. Private boats launch at Sorrento Boat Ramp (Point Nepean Road, Sorrento, all-tide concrete ramp with trailer parking) or Queenscliff Boat Harbour ramp (Larkin Parade, Queenscliff), roughly 15 minutes to the site.

The site is on the shallow reef just inside Corsair Rock on the Nepean side of the entrance, 12–15 minutes from Portsea; the reef breaks in any sea and the boat must stand off in deeper water. Skippers must call Lonsdale VTS (Point Lonsdale) or listen to VHF 12 for shipping movements before dropping divers, and must carry a dive flag; anchoring is not permitted in the shipping channel, so a live boat and a drop-shot or drift float is the normal method.

It is dived only on slack water at the end of the ebb (SWE); on other tides the current across the site is unworkable. Slack water at Port Phillip Heads does not coincide with high or low water: it falls roughly three hours after high or low water at Point Lonsdale, and the usable window is typically 20–40 minutes. Use the Ports Victoria / Port of Melbourne slack-water predictions for The Heads (also republished by charter operators and WillyWeather 'Rip Bank') rather than a standard tide table.

Being outside or right at the entrance, it is exposed to Bass Strait swell: anything over about 1.5 m from the south-west or south, or a strong southerly/westerly wind against the ebb, produces standing waves in The Rip and charters cancel. Northerlies flatten the entrance and give the best days. Visibility is best in late summer and autumn (Feb–May) when the water is clear and the westerlies have eased; winter brings 11–13 °C water and bigger swells but often the clearest days after a run of settled weather.

Heavy rain has little effect this close to the ocean. Scuba Doctor: 'extremely dangerous and difficult, and best at very low tide' – i.e. dead low water on a calm day with no swell, at the ebb slack. Any swell makes the site undiveable.

Live-boat drop in the lee at ebb slack on a flat day; descend onto the reef at 5–8 m and follow the debris – boilers, crankshaft, propeller shaft and hull plates – which lie in a rough north–south line, keeping the swell behind you; turn after 15–20 minutes, well before the flood, and ascend under a DSMB for pick-up clear of the rock.

No regular scheduled charter – dived from private and club boats (VSAG, Power House Underwater Club, Scuba Diving Melbourne) and occasionally by Dive Victoria Group or Red Boats when a group requests it.