The Adelaide River Mouth reef (AR4) is the easternmost of the four engineered reef complexes installed off the Top End in 2019, sitting in about 16 m of water off the mouth of the Adelaide River in Adam Bay, east of Gunn Point. The 2019 installation was part of an $8.3 million NT Government artificial reef project described at the time as the largest single installation of artificial reefs in the Southern Hemisphere, with a design life of well over 100 years. Each module is a purpose-built concrete structure about 5 m tall, 4 m square at the base and weighing about 24 tonnes, designed with openings and internal cells to provide shelter at several levels, and the modules are grouped into a reef complex rather than a single block.

The site was chosen for anglers: the Adelaide River is one of the Territory's great fish-producing rivers and the reef was placed to hold snapper, jewfish, cod and pelagics near its mouth. For divers, the setting is challenging. This is a turbid estuary-mouth environment with fine suspended sediment, strong tidal flows and, for much of the year, a plume of fresh water spilling from the river. Visibility is typically measured in a few metres at best.

On a rare clear late-dry-season neap, the modules would appear as tall grey towers on a mud and sand bottom, carrying sponges, hydroids and oysters rather than coral, with black jewfish, estuary cod and golden snapper around them and baitfish above. There are no published diver accounts and the site should be regarded as exploratory.

The Adelaide River is also one of the most crocodile-dense rivers in the world, and the risk around its mouth is higher than at any other artificial reef in the region.

The most practical launch is the concrete ramp at Leaders Creek, reached via the Arnhem Highway and Point Stuart/Leaders Creek roads (about 2.5 hours' drive from Darwin on part-gravel roads; check road status in the wet). From Leaders Creek run west along the coast to the Adelaide River mouth in Adam Bay. Alternatively, launch at Buffalo Creek (Lee Point) or the Gunn Point area and cross Shoal Bay and Adam Bay – a long run in open water.

Find the modules on the sounder and drop a shot beside them.

Heavily tidal estuary-mouth site. Only consider it on the smallest neap tides, at slack water, late in the dry season (August–October) when river flow is lowest; during and after the wet season the Adelaide River pours sediment-laden fresh water across the site and visibility is effectively zero. South-east trades make Adam Bay choppy by late morning.

Cyclone risk November–April; box jellyfish October–May.

If visibility allows, descend the shot to the modules at 14–16 m, keep physical contact with the shot or a reel line, and tour the nearest modules only. Abort if visibility is under 2 m. Ascend on the shot before the tide turns.

Modules are open concrete frames; do not enter the interior cells. Nitrox useful at 28 m sites.

No regular dive operator. Built for recreational anglers; divers visit only by private boat or on occasional club trips (Darwin Sub Aqua Club).