Portsea Back Beach Wall is the outside-the-Heads dive that does not depend on The Rip. It lies in Bass Strait off the ocean beaches of the Nepean peninsula, an east–west wall and scattered reef at 17–26 m (the wider area 12–30 m) whose soft calcarenite, in the Scuba Doctor listing's words, 'has eroded into lots of little swim-throughs, valleys and overhangs all topped off with a light scattering of kelp'. The listing places it 'just outside Port Phillip Heads on the Point Nepean side', running offshore from Portsea back beach.

What brings divers here is the fish. The listing describes the area as notably biodiverse, with blue devils, weedy seadragons, seals and abalone, and 'crayfish abound under ledges and often they are found in the open, walking around unafraid' – a sign of how lightly the site is fished. Over the wall the pelagics of the open coast appear in a way they rarely do inside the Heads: schools of snapper, yellowtail kingfish and barracouta have all been recorded, and the listing calls it 'a fantastic dive site for an avid fish photographer'. The encrusting life on the undercuts is the same zoanthid-and-sponge palette as the Heads walls, and the water is oceanic and clear.

The constraint is weather rather than tide. There is no channel and the current is moderate, so the dive can be timed for convenience, but the site is fully exposed to Bass Strait swell and the passage out and back through the Heads must be planned around The Rip. It is dived by the Portsea charters as an occasional calm-day 'back beach' trip and by private boats from Sorrento, and it suits Advanced divers who want a long, relaxed dive on an ocean reef with big fish, without the stopwatch of a slack-water wall. Toms Reef, 300 m to the south-east, is on the same ground.

Boat access only. From Portsea Pier or Sorrento Boat Ramp it is a 25–30 minute run west through the Heads and then east along the back beaches to a position off Portsea back beach and London Bridge; from Queenscliff allow 35 minutes. The site is not in a channel and boats anchor on sand beside the wall.

There is no shore access – the beach is 1.5 km away across open water and surf. Because it is outside the Heads the run home is against the ebb standing waves if the timing is wrong; skippers plan the passage back through the entrance for slack or the flood.

Not a slack-water site – there is no shipping channel and the tide runs at a moderate rate along the coast – but 'subject to swells and currents; very weather dependent' (Scuba Doctor). It faces Bass Strait squarely: any south-westerly swell above about 1.5 m or a southerly wind makes both the site and the passage through the Heads unworkable, and the best days are light northerlies in late summer and autumn, when visibility runs 15–20 m. The Heads themselves must be calm enough to transit in both directions.

Winter is possible in settled spells but the swell is rarely low enough.

Anchor on sand near the wall and descend to the base at 24–26 m. Follow the east–west face along its undercuts and through the little swim-throughs and valleys at 18–24 m, watching the open water for snapper, kingfish and barracouta schools and the sand edge for seadragons; return along the reef top at 17 m and ascend the anchor line or a DSMB.

Dive Victoria Group and Bayplay (Portsea) – occasional 'back beach' charter on calm days; Sorrento and Rye private boats; not tide-dependent so it is a common fallback when slack times are awkward.