Cockatoo Reef in the Pompey Complex holds one of the few blue holes on the Great Barrier Reef. Research on the Pompey reefs describes two such holes, at Cockatoo and Molar reefs, both roughly circular, 240-295 m across and 30-40 m deep, surrounded by lagoon depths of 5-10 m. They are interpreted as collapsed dolines (sinkholes) formed during low sea-level periods, then partly filled with sediment and colonised by coral as the sea rose after the last ice age.

At Cockatoo, the hole is completely rimmed by living coral, and its inner slopes drop at 60-70° to about 25 m before easing towards the floor. The Marine Life Network describes the Cockatoo blue hole as suitable for less experienced divers, lying inside a shallow 10 m lagoon that is exposed at low water, with limited fish life inside the hole itself. Staghorn and other corals cover the rim, and the outer reef supports sharks and pelagic fish.

Reaching the Pompeys is the main challenge: they lie far offshore from Mackay, in an area known for the fastest tidal streams on the reef, and there is no regular dive operator. Visits are by expedition charter or capable private boat, timed for neap tides and calm weather. The site is a geological curiosity as much as a dive, and it suits divers with Advanced certification who are part of a self-sufficient expedition.

The Pompey Complex is a line of 'hard line' reefs on the outer edge of the shelf roughly 150-200 km north-east of Mackay. Access is by expedition charter or capable private vessel from Mackay Marina (Mackay Harbour) or Hay Point; allow most of a day for the crossing in good weather. Navigation in the Pompeys requires great care: the complex has some of the fastest tidal streams on the Great Barrier Reef, and passages between reefs run hard on the tide.

The blue hole lies inside Cockatoo Reef's shallow lagoon and is visible from the air as a dark blue circle; boats anchor outside or in the lagoon at high water and divers enter at the rim.

The Pompey reefs are completely exposed to south-east trade winds and to the very strong tidal streams that sweep through the complex; spring tides can produce currents of several knots in the passages. Plan visits for neap tides and calm spells (typically September-December), and time dives inside the lagoon for the high-water slack when the reef is covered. At low water the lagoon rim is exposed.

Cyclone season November-April.

Enter at high-water slack on the lagoon side, descend over the coral rim and follow the inner slope (60-70°) down to a planned depth of no more than 25-30 m, then work back up the wall and finish over the staghorn on the rim.

The floor at 30-40 m is beyond recreational limits for many divers; plan conservatively given the extreme distance from a chamber. Sediment on the floor silts easily.

No regular operator – expedition charters and private vessels only; occasional Mackay-based charters by arrangement.