Weed Reef is a shallow harbour reef named after the sargassum seaweed that grows on it, and that weed is most visible around August, in the middle of the dry season. DSAC describes the site as a shallow dive with several large bommies, and marinelife.org.au singles out one large bommie with an overhang full of colourful life. Darwin's natural reefs are not the clear-water coral gardens of the Great Barrier Reef. The harbour is a macro-tidal, turbid system, and its reefs are low rock platforms and bommies covered in sponges, soft corals, gorgonians, sea whips and hardy hard corals that tolerate silt and big tidal flows. What they lack in visibility they make up for in density of life and the chance of turtles, rays and big fish.

The overhang is the heart of the dive: its roof and walls are covered in sponges, soft corals and ascidians, and fish shelter in the shade. Around the bommies the rock and rubble support more sponges and gorgonians, and small reef fish are everywhere. Depths of 12-16 m mean long bottom times, so it is a good second dive after a wreck, and an easy introduction to Darwin reef diving for Open Water divers. Visibility varies with the tide and season like everywhere in the harbour, and on a good neap in the dry season it can be pleasant. Photographers should bring a macro setup for the invertebrate life in the overhang, and a torch helps bring out colour. The sargassum itself changes the look of the reef through the year, dense in the middle of the dry season and much thinner in the wet.

Boat only. Launch at Cullen Bay Marina or the Cullen Bay public ramp (off Marina Boulevard, 5 minutes from the CBD, trailer parking) and run out into the harbour; most harbour reefs are 10-25 minutes away on a calm neap morning. Positions are known to local operators and DSAC rather than widely published, so go with someone who has the mark or use a good chart and sounder.

Anchor on sand beside the reef, not on coral, and plan to arrive before high-water slack.

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 beside the largest bommie at neap slack, circle it looking under the overhang, then visit the neighbouring bommies. Long bottom time is possible at 12-14 m; ascend on the anchor line.

Darwin Dive Centre and Dive Darwin use the harbour reefs for reef dives and training on neap-tide charters, often as a second dive after a wreck; Darwin Sub Aqua Club dives them on club days. No daily schedule.