London Bridge is the famous sandstone arch of the Portsea back beach, a calcarenite span left standing on the wave-cut platform below 30–40 m bluffs, and the diving is on the reef and gullies that surround it. The Scuba Doctor lists it as a 5–7 m shore dive for Advanced divers, and the depth is misleading about the difficulty: this is a Bass Strait rock-platform entry where the sea, not the diver, decides whether a dive happens. On the rare flat days the site is superb.

The intertidal flats either side of the arch are cut by gullies that lead out to broken limestone reef, undercut into ledges, small caves and swim-throughs by the same weathering that carved the arch, all roofed with kelp and crusted beneath with sponges, bryozoans and jewel anemones. The shallow bay to the west holds gullies full of fish, and the offshore reef carries schools of sweep and old wives, zebrafish, magpie perch, blue-throated wrasse, leatherjackets and long-snouted boarfish, with southern rock lobsters and abalone in the crevices and Port Jackson sharks and draughtboard sharks resting under ledges. Weedy seadragons drift in the kelp, and fur seals from the Heads occasionally investigate divers.

The site's history is grim – the 1967 disappearance of Prime Minister Harold Holt while swimming at Cheviot Beach a kilometre west is the standard warning about this coast – and its exposure is total: it is rip-prone, surgy even on calm days, and within reach of the tidal streams of The Rip. It belongs to experienced divers with surf-entry skills who have watched the sea for a while before gearing up, ideally with someone who knows the gullies. For them it is one of the most dramatic shallow dives in Victoria; for everyone else it is a place to snorkel the inner gullies at low tide on a glassy day, or to admire from the lookout.

From Portsea village take Back Beach Road and then London Bridge Road south to the car park at its end (Mornington Peninsula National Park). A steep ramp leads down to the beach – 150 m or so and a hard climb back with gear. The best entry is from the rock platform east of the arch at low tide, when the intertidal flats are exposed and the gullies between them are sheltered; the shallow bay west of the arch is the alternative in the calmest weather.

Exit by the same gully; there is no alternative exit for several hundred metres.

The back beaches face south-west into Bass Strait and are safe to dive only in rare flat conditions: The Scuba Doctor's rule of thumb is swell under about 1.5–2 m with periods of 10 seconds or more, usually only after three or four days of northerly to north-easterly winds, with light northerly winds on the day. Any southerly or westerly wind, or a fresh swell, makes the entries lethal. Dive around low tide when the limestone platforms are exposed and the gullies are sheltered, and abort if there is white water over the entry.

Rips and tidal streams near The Rip run even on calm days, and conditions change within an hour. Winter high-pressure systems bring the calmest spells; summer sea breezes rule out afternoons.

Enter from the eastern platform at low tide, drop into the gully in 3 m and work out through the gullies to the offshore reef in 5–7 m, keeping the arch as a reference. Explore the ledges and small caves, turn after 20 minutes or at half gas, and return through the same gully with the surge, timing the exit between sets.

No regular operator. The Scuba Doctor (Rye) and Peninsula clubs run occasional guided shore dives when conditions allow; otherwise experienced private groups only.