Petaj is one of the best-known moorings on Milln Reef and a staple of both day boats and the Pro Dive Cairns liveaboard. The dive follows a wall that runs down to around 20 m, with a swim-through part-way along it that is large enough for a relaxed passage and makes a natural highlight of the dive. Along the base, thickets of staghorn coral grow out across the sand, and fan corals decorate the wall face.

Pro Dive Cairns notes that Petaj has resident turtles and whitetip reef sharks, and that is its main drawcard: turtles are seen on most dives, often resting on the sand or under ledges, and the whitetips patrol the base of the wall or lie in the swim-through. The general fish life is rich, with clownfish, batfish, coral trout, cod, fusiliers, butterflyfish, angelfish and basslets, and schools of barracuda sometimes hang off the wall. At night the site is excellent: parrotfish sleep in mucus cocoons in the crevices, turtles doze under overhangs, and barracuda and whitetips hunt along the reef.

The moderate depth and reliable conditions make Petaj suitable for newly certified divers, while the swim-through and resident animals keep it interesting for experienced divers on repeat visits. It sits close to Three Sisters, Swimming Pools and the Desert area, so guides sometimes combine them. Snorkellers can see the upper wall and the occasional turtle when the water is calm, although most of the action is below 8 m.

Milln Reef lies about 55 km east of Cairns, roughly 76 minutes by fast boat, and is one of the core reefs of the southern Cairns group.

Boat access only. Cairns day boats leave from the Reef Fleet Terminal (1 Spence Street) and the neighbouring Marlin Marina on the city waterfront, with check-in between about 7:00 and 8:00 am; most operators run transfers from city and northern-beaches accommodation, and there is paid parking near the terminal. The run of about 55 km out through Trinity Inlet and across the lagoon to Milln Reef takes around 76 minutes on a fast catamaran and noticeably longer on slower dive boats or into a south-easterly chop.

Petaj is a Silverswift and Pro Dive Cairns mooring; it is part of the Three Sisters/Swimming Pools cluster on Milln. Enter from the stern platform.

The prevailing south-east trade winds (typically 15-25 knots from April to September) are what limit Cairns outer-reef diving: above about 25 knots trips may be cancelled, and exposed eastern moorings are used only in light winds. Summer northerlies are gentler but make north-facing moorings choppy. Box jellyfish and Irukandji are present from about November to May (stinger suits on every dive), and the cyclone season runs November to April, when a system in the Coral Sea can close the reef for days.

Visibility drops after heavy rain and strong winds; spring is usually calmest and clearest. Petaj is on a sheltered part of Milln and is used in moderate south-easterlies; it is a regular liveaboard night mooring.

Descend to the base of the wall at 15-18 m, follow it to the swim-through (the guide will point it out), pass through, then turn and return shallower along the wall at 8-10 m. Check the staghorn beds on the sand for turtles before your safety stop at 5 m.

Silverswift – daily day trips (Milln is one of its core reefs); Passions of Paradise – daily sailing-catamaran trips to Milln moorings; Pro Dive Cairns – 3-day/2-night liveaboard including Milln night dives