Montague Island has its own Bubble Cave, one of several in NSW. McFadyen mentions it close to Pebbly Bay near the seal colony, and the Marine Life Network describes it as a 5–30 m site on the northern headland reef with an air pocket where divers' bubbles accumulate. It warns that the air in the pocket may be poor quality – it is exhaled gas, low in oxygen and high in carbon dioxide – so divers should keep regulators in their mouths.

The cave is part of the boulder and rock reef of the northern headland, where large granite blocks form overhangs and gaps. The cave walls carry sponges, sea squirts and encrusting algae, and fish such as eastern blue devilfish, bullseyes and cardinalfish shelter inside. The reef outside ranges from kelp-covered shallows to sponge-covered rock at depth, and the proximity to the seal colony means seals often join divers. Blue groper, sweep, yellowtail and red morwong are common, and grey nurse sharks aggregate on the northern side of the island in summer.

The site suits Open Water divers who stay shallow and visit the cave, with deeper options for Advanced divers. Current can run around the northern end of the island, so the operator will choose the timing. Coordinates here are approximate. Specify Montague Island when referring to it, to avoid confusion with the Black Rock Bubble Cave at Batemans Bay or the North Solitary Island site. Operators decide on the day whether the cave is diveable, depending on swell and current.

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 cave is in the reef off the northern headland of the main island, close to the seal colony and the Pinnacles.

Montague Island is about 9 km offshore and fully exposed, but with sites on all sides operators can usually find shelter from the wind of the day: the northern bays in southerlies, the western side in easterlies. The real limit is the Narooma bar, which closes in moderate easterly or southerly swell, especially on a run-out tide; McFadyen calls it probably the most dangerous bar in NSW. Trips are frequently delayed or cancelled, so allow spare days.

Currents are strong on the northern and eastern sides. The northern headland is exposed to current and swell, so the site needs a calm day; surge makes the cave uncomfortable.

Descend on the northern headland reef, find the cave entrance and surface briefly into the air pocket, keeping the regulator in. Then explore the reef outside, choosing a depth between 10 and 25 m, and return to the boat.

Dived by Narooma operators as conditions allow; listed by the Marine Life Network and mentioned by McFadyen.