Portsea Back Beach is the ocean side of the Mornington Peninsula, facing straight south into Bass Strait. The reef runs from the shore out to around 14 m and holds crayfish, abalone, blue devils, banjo sharks, weedy seadragons and big schools of sweep over the kelp. It's a shore entry through the surf so you need a flat day — south swell punches straight in here. Diveable conditions are light northerlies (offshore winds flatten the surface and lift the visibility), swell under a metre, and ideally near low slack tide. Park at the Portsea Back Beach surf lifesaving club and walk down to the water. Use the live 7-day wind and swell forecast below to pick your day. Intermediate divers only — surf entry, exposed coast, no easy bail-out if it picks up while you're down.
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.