The Catacombs on Llewellyn Reef is described by marine life writers as a wall riddled with caves, ledges and swim-throughs, running from about 12 m to 30 m. It is one of the more dramatic sites in the northern Bunker Group and a reward for those with a boat capable of reaching it. The wall is richly decorated with soft corals, gorgonians, black coral trees and sea whips.

Angelfish, parrotfish, surgeonfish, butterflyfish and coral trout move along it, turtles and reef sharks are common, and pelagic fish and mobula rays pass in the blue. The swim-throughs and ledges add a sense of exploration, and a torch brings out the colour inside them. Because the site is remote and deep, it suits divers with Advanced certification and good buoyancy.

The swim-throughs should only be entered after checking that they are open and wide, and none should be treated as cave penetration without appropriate training. A shallower site on the same reef, Trout Spot, offers a coral garden suitable for a second or night dive, which makes Llewellyn a good overnight anchorage stop on a Bunker Group cruise. The reef should not be confused with the SS Llewellyn, a historic wreck off Mackay, which is a different site entirely.

The Bunker Group reefs lie 80-100 km north-east of Bundaberg and 50-90 km east of the Town of 1770. Private boats launch from the 1770 Marina or Round Hill Creek ramp (bar crossing – check tides), or from Burnett Heads (Bundaberg Port Marina); expect 1.5-4 hours depending on vessel and destination. Yachts commonly anchor overnight in the Lady Musgrave or Fitzroy Reef lagoons and dive the surrounding reefs by tender.

Llewellyn Reef lies in the northern Bunker Group between Fitzroy and Boult reefs; anchor in the lee in settled weather.

The Bunker reefs are open ocean: south-east trades over about 20 knots make the crossing from 1770 or Bundaberg rough and limit diving to the lee (north-western) sides, while the Fitzroy and Lady Musgrave lagoons provide secure anchorages. The best windows are September-December and March-May calms. Tidal currents run strongly around reef edges and through lagoon entrances on springs.

Cyclone season November-April; summer plankton reduces visibility.

Descend to the top of the wall at 12 m, then work down to 20-25 m along the wall, passing through the open swim-throughs only after checking them. Turn with ample margin, ascend along the wall and finish on the reef top.

No scheduled day trips. Lady Musgrave Experience mentions the northern Bunker reefs (Boult, Llewellyn, Lamont and Fitzroy) as remote sites for special or extended trips, Heron Island Resort has historically run occasional long-range trips to the northern Bunker reefs, and most diving is from private yachts and occasional liveaboards.