Nepean Wall is the eastern counterpart of Lonsdale Wall, the cliff that forms the Point Nepean side of the drowned river channel at the mouth of Port Phillip. The Scuba Doctor listing describes a boomerang-shaped trench just outside the Heads whose dramatic northern wall falls to 65 m, with the best diving between 18 and 30 m. The face is vertical, and it is cut with expansive caves and undercuts – bigger, on the whole, than the overhangs on the Lonsdale side – all of them hung with soft corals, sponges and zoanthids fed by the full tidal exchange of the bay.

The dive is run from a shot on the 18 m reef top; divers swim south to the lip and drop onto the face, then work along it at 25–30 m in and out of the caves. Fish life is described as prolific, both reef fish – blue devils in the caves, boarfish, old wives, magpie perch, wrasse – and pelagics in the open water off the wall, with southern rock lobster in the ledges and Australian fur seals from the Point Nepean haul-out frequently joining divers. Sevengill sharks are seen in the deeper water.

What makes Nepean Wall more demanding than the central Lonsdale sections is timing. It sits outside the Heads proper on the seaward side of the entrance, so the slack is shorter and the return of the current more abrupt; the listing's route notes stress returning to the shot for the ascent and keeping bottom times short. The convention is to dive it on the ebb-to-flood turn so that the new flood sets divers back into the bay rather than out into Bass Strait. The wall continues to 65 m and, with Trimix Corner 28 m away, it is also a technical destination. For Advanced divers who have done the Lonsdale wall and want the bigger caves and the more exposed feel of the Nepean side, this is the next step.

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 just outside the Heads off Point Nepean, on the eastern side of the entrance; from Portsea Pier it is a 10–15 minute run west past the Point Nepean quarantine station. 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, either at the end of the flood (SWF) or the end of the ebb (SWE); most operators prefer the flood slack so that any residual current sets into the bay rather than out to sea. 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. Operators time the dive for the ebb-to-flood turn so that the first of the flood carries divers back into the bay; being outside the Heads it is dropped whenever the entrance is breaking.

Shot is dropped on the reef top at 18 m a few minutes before the end of the ebb. Descend and swim south from the shot to reach the northern wall of the trench, drop over to 25–30 m and follow the vertical face with its big caves and undercuts. Keep the bottom time short – 20 minutes – and return to the shot for the ascent, as the current turns fast here; if it has already turned, ascend under a DSMB and let the young flood carry you inside.

Base 65 m; Trimix Corner adjoins. Trimix below 40 m; drift deco under DSMB with tracking boat; flood-turn timing essential so deco drifts inside.

Dive Victoria Group (Portsea and Queenscliff) – double boat dives to the Heads walls most days of the week, site chosen on the morning by slack-water time and swell; Bayplay Adventure Tours (Portsea) – weekend and holiday charters; Red Boats (Queenscliff) – weekend Heads charters; Melbourne clubs (VSAG, Power House Underwater Club, Scuba Diving Melbourne) run their own boats here regularly.