The Petriana holds an unhappy place in Australian history: her loss produced the country's first major oil spill. She was an iron screw steamer of 1,821 tons and 79 m, built by A. Leslie & Co at Newcastle upon Tyne in 1879 as a cargo ship and converted to a tanker in 1891, by 1903 operated by the Asiatic Petroleum Company, the Shell–Royal Dutch joint venture. On 28 November 1903, carrying 1,300 tons of petroleum from Borneo, she was brought in toward the Heads in thick fog by pilot Henry Press and ran onto the reef on the ocean side of Point Nepean – 'aground on Portsea Back Beach at 7 a.m.' Salvage failed, and to lighten her the cargo was pumped into the sea, fouling the peninsula beaches.

The Scuba Doctor listing describes the remains as lying 'in pieces around 200 metres offshore', with 'the bow relatively intact in 3 metres of water' and the rest scattered over the reef to about 6 m; the wreck 'can be heavily sanded over at times' and the site is subject to 'strong cross currents' and surge. What shows above the sand – plating, frames, the bow section – is covered in kelp and coralline algae, with crayfish and abalone in the iron and sweep, wrasse and zebrafish over the reef, and the clear oceanic water of the Nepean back beaches.

It is an ocean-side site on a surf-swept reef with no shore access, dived rarely by club boats on the flattest ebb-tide days, and its interest is historical: a protected historic shipwreck that marks both the hazards of a fog-bound pilotage and the beginning of the oil-spill era in Australia. Sweep Reef and Beacon Rock lie along the same system to the west.

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 15 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 20 minutes to the site.

The site is on the ocean side of Point Nepean off Portsea back beach, reached by boat out through the Heads and east along the back of the point (15–20 minutes from Portsea/Sorrento); shore access from the national park is not permitted and the surf makes a beach entry impractical. 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 on the ebb slack or a gentle ebb only, when the outgoing tide leaves this side of the reef in relatively still water. 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. The listing notes 'strong cross currents' and surge on the site, and that the wreck 'can be heavily sanded over at times'; a flat, northerly day on the ebb, ideally after storms have shifted the sand. Tide reference WillyWeather 'Petriana Reef'.

Live drop on the ebb in the lee of the reef, descend to the relatively intact bow in 3 m and work seaward over the scattered plating and frames to the sand edge at 6 m, then back along the reef; keep the swell behind you, keep it short and ascend under a DSMB.

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.