The John Holland Barge is the anchor structure of the harbour's refugee-boat reef. It was a steel construction barge from the John Holland company, built around a row of cylinders about 2 m in diameter connected by heavy steel beams that once carried a working platform, a deckhouse and a small work boat. DSAC gives its size as 18 by 12 m and the year sunk as 1982; marinelife.org.au describes it as a steel dumb barge of over 200 ft sunk in July 1984. Between 1976 and 1981 more than 50 boats carrying Vietnamese refugees reached Australia's northern coast, most of them arriving in Darwin Harbour. Several of the steel boats were later cleaned and scuttled west of Cullen Bay together with a redundant construction barge to create a fishing and diving reef, and the site was extended in later years with a confiscated illegal fishing boat. The group sits in 20-25 m within a few hundred metres, and some operators have linked the wrecks with rope so that more than one can be visited on a dive.

The barge landed upside down on the bottom in 21-25 m, so divers now see the cylinders and the web of beams from below, creating a maze of overhangs and gaps rather than a flat deck. That shelter is what the fish like. Glasseyes hang in the shadows, rays and wobbegongs rest under the beams, and angelfish, snapper, bream, cod, jewfish, trevally and octopus are all recorded. It is a good site for photographers looking for structure and for divers who enjoy poking into nooks with a torch without entering an enclosed wreck. With the Song Saigon and Ham Luong within easy reach, it is typically the middle stop of a multi-wreck dive on the complex.

Boat only. The refugee-boat complex lies in the outer harbour a short run west of Cullen Bay - typically 10-15 minutes from Cullen Bay Marina or the Cullen Bay public ramp (off Marina Boulevard, trailer parking on site). NT Fisheries publishes the positions of each wreck for anglers, so expect fishing boats on the marks on neap weekends.

Drop a shot beside, not into, the structure, and plan to be on site before high-water slack; the complex is exposed to the full tidal flow once the tide turns.

Tide is everything in Darwin Harbour: the spring range is 7-8 m and tidal streams run hard through the harbour, so diving is generally only possible on neap tides (range under about 3-4 m), timed to the slack at high water when clearer ocean water sits over the site. Spring tides bring strong currents and silt-laden water with near-zero visibility. The dry season (May-October) gives the most consistent conditions; the south-east trade winds can chop up exposed water in the afternoon, so mornings are best.

In the wet season (November-April) run-off from the rivers and creeks drops visibility, afternoon storms build quickly, and the harbour is in cyclone season; box jellyfish are present October-May.

Descend the NT Fisheries mark to the upturned barge. Swim along the row of large cylindrical pontoons, peer into the gaps under the steel beams of the old working platform for resting rays and wobbegongs, then circuit the perimeter. Link to the refugee boats if a line is in place; return to the shot.

Darwin Dive Centre and Dive Darwin include the Vietnamese refugee boats and artificial-reef wrecks in their neap-tide harbour charters; Darwin Sub Aqua Club dives them regularly on club days. No fixed daily schedule - trips follow the neap cycle.