Yellowfin Alley is a deep reef at Montague Island listed by the Marine Life Network at 15–35 m. It describes excellent fish life, colourful sponge gardens and sea pens on the sandy areas. The name hints at the pelagic fish that gather around Montague in summer: yellowfin tuna are caught offshore from Narooma, and schools of kingfish, bonito and trevally commonly work the reefs around the island.

The reef has the typical deep Montague profile: kelp on the upper parts, then rock covered in sponges, gorgonians, sea whips and sea squirts. The sand at the base of the reef supports sea pens, soft corals that stand up out of the sand and retract when disturbed, along with rays and flathead. Fish include schools of yellowtail, one-spot pullers and silver sweep, red morwong, blue groper and nannygai, with pelagics in the blue.

The depth range makes this an Advanced dive, and there is no published position or detailed description beyond the Marine Life Network's summary. Dive with a Narooma operator and follow their briefing. It is a site to look for if you have already done the seal and grey nurse dives and want to see Montague's deeper sponge gardens. Visibility is often good when warm, blue water from the East Australian Current pushes in during late summer and autumn, which is also when the pelagic fish are most active. Keep an eye on depth when watching the sea pens, because the sand edge is the deepest part of the dive and it is easy to drift deeper while photographing them.

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 site's position around the island is not published; operators choose it 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.

Descend onto the reef at 15–20 m, drop to the reef–sand edge at 25–30 m to look for sea pens, then work back up through the sponge gardens to the reef top for the ascent.

Listed by the Marine Life Network; dived occasionally by Narooma operators.