Twin Bommies, also known as Twin Towers Reef, is the third of the outer Lonsdale reef's named bommie sites, lying between Lonsdale Arches and Lonsdale Bommies about a kilometre off Point Lonsdale in Bass Strait. The Scuba Doctor listing describes 'two substantial rock formations rising from an 18-metre seafloor', with 'abundant fish communities' and 'vibrant soft corals and sponges covering the bommies', and rates it Open Water at 12–18 m, ebb tide only.

The two towers stand close enough together to be dived as one site, each a steep-sided limestone stack 5–6 m high with undercut bases, ledges and splits, and the tidal flow through the entrance has coated them in the same soft corals, zoanthids and sponges as the Arches next door. The listing singles out Port Jackson sharks, which rest under the ledges at the base of the towers in numbers in the cooler months, and southern rock lobster in the cracks; the fish over and around the stacks are the outer-reef community of sweep, boarfish, magpie perch, old wives and wrasse, with cuttlefish in winter and seals passing from the Point Lonsdale haul-outs. It is, the listing says, an excellent photographic site, and the two towers give a diver a subject with sky behind it in a way that a flat reef cannot.

It is dived on the ebb on calm days, like the whole outer reef, and is usually combined with the Arches or the Bommies on an 'outside' charter day from Queenscliff or Portsea. For an Open Water diver it is a straightforward and memorable Bass Strait dive; the only requirements are a flat sea, the right tide and a boat that will pick up a DSMB downstream if the group drifts off the towers.

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 20 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 20 minutes to the site.

The site is on the outer Lonsdale reef between the Arches and Lonsdale Bommies, reached through the entrance on the ebb; a second 'mystery' mark east of Castle Rock (-38.298367, 144.600867) awaits verification. 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 on the ebb slack or a gentle ebb only, when the outgoing tide leaves this side of the reef in relatively still water. 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.

Being outside or right at the entrance, it is exposed to Bass Strait swell: anything over about 1.5 m from the south-west or south, or a strong southerly/westerly wind against the ebb, produces standing waves in The Rip and charters cancel. Northerlies flatten the entrance and give the best days. 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. Open Bass Strait exposure: dropped in a south-westerly swell above about 1.5 m.

Shot or anchor between the two towers at 15 m on the ebb slack. Circle each formation from the 18 m sand to its top at 12 m, checking the undercuts for Port Jackson sharks and crayfish and the faces for soft corals, then cross to the other; the Arches are 80 m west if time and slack allow. Safety stop on a tower top or under a DSMB.

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.