The Keyhole is described by the Marine Life Network as one of the best deep sites at Montague Island. It lies west of the seal colony, with deep sponge gardens from 20 to 40 m featuring large sea fans and schools of pelagic fish. That places it in the zone below the kelp, where rock surfaces are covered in filter feeders and the open-water location attracts fish.

Expect large gorgonian sea fans, sponges of many shapes and colours, sea whips and sea squirts, with bullseyes, nannygai and red morwong on the reef and schools of yellowtail, silver trevally and kingfish in the water column. Being close to the seal colony, divers often have seals for company, and grey nurse sharks from the northern gutters may pass through in summer. Visibility is often good at this depth when the East Australian Current pushes warm blue water south.

This is a deep dive: at 30–40 m, air no-decompression limits are very short and narcosis is significant, so it suits Deep-certified divers with experience, ideally on nitrox or with technical training for the deeper parts. The name suggests a hole or gap in the reef, but no published description explains it. The position is not published, so dive with a Narooma operator. Sharks, including great whites, are occasionally seen near seal colonies, so follow the operator's briefing. Because the site is used only when conditions are right, ask the operator in advance whether a deep day is planned and bring nitrox certification if you have it. A torch or strobe brings out the reds and oranges of the sea fans, which look grey-blue in natural light at this depth.

Boat access only. Narooma Charters leaves from the Narooma town wharf on Wagonga Inlet (about 25 minutes to the island) and Underwater Safaris from Narooma marina; private boats launch at the Apex Park or Bar Beach ramps inside the inlet. Every trip crosses the Narooma bar, one of the most dangerous on the NSW coast, so departures depend on bar conditions; Bermagui harbour (about 24 km by sea) is the fallback launch point.

The Keyhole lies west of the seal colony at the northern end of the island; it is a deep site that operators choose for experienced divers on calm days.

Montague Island is about 9 km offshore and fully exposed, but operators can usually find a sheltered side. The main limit is the Narooma bar, which closes in moderate easterly or southerly swell, especially on a run-out tide, so trips are often delayed or cancelled. Currents are strongest around the northern and eastern sides.

A deep site near the northern end needs little current and light winds, so that divers can descend and recover safely.

Descend the line directly to the sponge garden at 25–30 m, explore within easy reach of the line, watching the blue for pelagic schools, and begin the ascent early enough to stay within no-decompression limits.

Nitrox for the shallower range; trimix/decompression training for the 40 m sections.

Listed by the Marine Life Network as one of Montague's best deep sites; run by Narooma operators for experienced divers when conditions suit.