London Bridge at Portsea is the natural rock arch on the back-beach side of the Mornington Peninsula, with reef diving out to around 15 m on the seaward side. The site holds crayfish, abalone, weedy seadragons, blue devils and a thick kelp canopy that opens into sand patches further out. Access is via the steep stairs from the London Bridge car park then a swim or short scramble to the entry — it's not a casual dive, the surf zone bites if the swell is up. Diveable in light northerlies with south swell under a metre and tide near slack. The arch itself is a gorgeous landmark to surface near. Use the live 7-day wind and swell forecast on this page to plan around the conditions. Intermediate to advanced shore divers — exposed coast, surf entry, plan a flat day.
How far you can see underwater — measured in metres. 10m+ is great, 5–10m is workable, under 3m is murk. Driven by wind, swell, and recent rain.
Long-period waves rolling in from the open ocean. Direction matters more than height — a S swell hits Portsea hard, but an E swell rolls past. Period over 12 s = real ocean punch.
Offshore (N or NE) flattens the surface and clears the water. Onshore (SE through SW) chops it up and stirs sand. Calm or light offshore is the magic combo.
This site faces the open ocean. The exposure caption above shows which directions slam in. Anything from the opposite side gets blocked — that's the safest window.
Slack water — the 30 minutes either side of high or low — is calmest and clearest. Mid-tide brings the most flow. Plan to be down at slack, up before the run picks up.
Bass Strait sits 14–16 °C autumn–winter, 17–19 °C summer. Below 16° a 7 mm hooded keeps you warm for 60 min+. Drysuit if you're going long.