12lb Reef is one of the two Lonsdale Bight sites named for crayfish, and the story is in the name: a twelve-pound southern rock lobster – around 5.5 kg, an enormous animal – was taken from a cave in this wall in the days before the marine national park closed the reef to fishing. The Scuba Doctor listing gives a depth range of 8–20 m, an Open Water rating and the standard slack-water restriction, and describes the site simply: 'the many overhangs and plentiful soft corals make this a popular and picturesque dive', with large crays still to be seen in the overhangs.

The wall is the eastern edge of the Lonsdale Wall system where it fades into the sponge reefs of the bight – a drop of 8–12 m from a reef flat at 8 m to sand and rubble at 18–20 m, cut into overhangs and small caves along its length. The overhangs are hung with soft corals, yellow zoanthids and sponges, and the caves hold the crayfish, blue devils and bullseyes that the shelter attracts; over the top, old wives, magpie perch, leatherjackets and wrasse work the kelp and sponge garden. The Grotto, a cave site, is 36 m away and Boarfish Reef, with its anchor and 400 m sponge garden, is 66 m the other way, so a single slack can take in all three.

Because it sits within Open Water limits and is inside the Heads, 12lb Reef is one of the sites the Queenscliff charters use for less experienced divers on a day when the walls are running or the outer reef is rough, and for photographers who want soft-coral overhangs without depth. It is dived on both slacks. 16lb Reef, its namesake on the western side of the system near Foggy Reef, is a separate site.

Boat access only. Charter boats leave from Portsea Pier (Dive Victoria Group, Bayplay) and Queenscliff Harbour (Dive Victoria Group, Red Boats); run time is about 12 minutes. Private boats launch at Sorrento Boat Ramp (Point Nepean Road, Sorrento, all-tide concrete ramp with trailer parking) or Queenscliff Boat Harbour ramp (Larkin Parade, Queenscliff), roughly 10 minutes to the site.

The site is in Lonsdale Bight next to Boarfish Reef, 10 minutes from Queenscliff Harbour and 15 from Portsea; the boat usually shots Boarfish Reef and divers cover both. Skippers must call Lonsdale VTS (Point Lonsdale) or listen to VHF 12 for shipping movements before dropping divers, and must carry a dive flag; anchoring is not permitted in the shipping channel, so a live boat and a drop-shot or drift float is the normal method.

It is dived only on slack water, either at the end of the flood (SWF) or the end of the ebb (SWE); most operators prefer the flood slack so that any residual current sets into the bay rather than out to sea. Slack water at Port Phillip Heads does not coincide with high or low water: it falls roughly three hours after high or low water at Point Lonsdale, and the usable window is typically 20–40 minutes. Use the Ports Victoria / Port of Melbourne slack-water predictions for The Heads (also republished by charter operators and WillyWeather 'Rip Bank') rather than a standard tide table.

Inside the Heads it is sheltered from most swell, but a south-west swell over about 2 m still surges across the top of the wall and a westerly or south-westerly wind over 15–20 knots makes the boat work unpleasant; big seas outside drop visibility to a few metres for a day or two. Visibility is best in late summer and autumn (Feb–May) when the water is clear and the westerlies have eased; winter brings 11–13 °C water and bigger swells but often the clearest days after a run of settled weather. Heavy rain has little effect this close to the ocean.

Shot onto the reef top at 8–10 m at slack. Drop over the short wall to 15–20 m, follow the overhangs along its base with a torch for the soft corals and the crayfish caves, and return along the top; The Grotto and Boarfish Reef are within a few fin-kicks and can be included. Safety stop on the reef top, DSMB for pick-up.

Dive Victoria Group (Portsea and Queenscliff) – double boat dives to the Heads walls most days of the week, site chosen on the morning by slack-water time and swell; Bayplay Adventure Tours (Portsea) – weekend and holiday charters; Red Boats (Queenscliff) – weekend Heads charters; Melbourne clubs (VSAG, Power House Underwater Club, Scuba Diving Melbourne) run their own boats here regularly.