The Pinnacle, also known as Platters Rock, is a large ridge of rock that rises from about 15 m to within 5 m of the surface. DSAC notes good coral growth and fish life, and marinelife.org.au gives a fuller picture: soft corals, sponges and gorgonians cover the rock, with nudibranchs and octopus in the crevices and turtles passing by. 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 shape of the reef organises the dive. Larger fish - cod, snapper and trevally - tend to hold near the base, while the summit attracts clouds of small fish that swirl around the top of the ridge. The shallow top means divers can finish the dive with a long, interesting safety stop, and on a clear neap the top can be seen from the surface, which also makes it a candidate for snorkelling at slack water. Depths of 5-15 m give long bottom times, so it is a popular second dive and training site. The combination of vertical relief, invertebrate life and fish makes it one of the more varied natural dives in the harbour, and a good choice for macro photographers who want subjects at several depths.

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. The top of the pinnacle at 5 m shows well on a sounder.

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 to the base at about 15 m to look for the larger fish, then spiral up the ridge, finishing on the summit at 5 m among the clouds of small fish - a natural safety stop.

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.