Gunnamatta is a long open south-west-facing beach on the Mornington Peninsula's surf coast, better known to surfers than divers. The reef runs out to around 16 m and holds crayfish, abalone, weedy seadragons and the usual sponge-and-kelp cover, but conditions are demanding — Gunnamatta cops swell from anywhere south through south-west and rarely sits flat. You need a genuinely calm day: swell under a metre, light north-east winds, tide near slack. The shore entry punches through serious surf so most dives here are boat-launched from Rye or Sorrento on the bay side then run round. Use the live 7-day wind and swell forecast below to spot the rare flat window. Advanced divers only — strong currents, big surf, exposed coast. If it's not glass-flat in the forecast, dive somewhere else and come back tomorrow.
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.