The Trio is a puzzle of a wreck: a big ferro-cement barge about 50 m long, name unknown, which local lore says was being towed from Pittwater to Sydney Harbour when it foundered with a cargo that included a concrete yacht. It was found on 15 July 1988 by Bob Mansfield and two companions and named for the three of them. Sydney Dive Charters record it as a hopper barge lost at an unknown date.

It lies upright about 2 km off Mona Vale Beach on sand that shelves from 52 to 54 m, with the deck about 5 m off the bottom at roughly 46 m, making it the northernmost of the deep wrecks between Long Reef and Broken Bay and one of the least visited. The layout is simple. A large winch and a row of bollards mark the bow.

Three holds open along the deck: the first once held a deteriorating ferro-cement yacht, now mostly gone, and the other two are empty. At the stern is a steering compartment, memorable chiefly for containing a toilet, and a below-deck compartment forward of it. The midship hull has started to break down, and a hole into the aft hold gives access to the small power-plant space.

Because the whole structure is concrete over steel mesh rather than plate, it has held its shape better than the steel barges at Long Reef but offers little marine growth. Fish life is moderate, with bullseyes and jewfish in the holds. The Trio's interest is novelty and depth: it is a worthwhile change for teams who dive the Long Reef group regularly, worth a dive every now and again rather than a destination in itself.

It suits experienced decompression divers on trimix with a competent boat handler, and anchoring is awkward unless the hook is placed directly on the deck.

Boat only, 2 km off Mona Vale Beach and about 4 km north of Long Reef. Boats launch from the Pittwater ramps (Bayview/Careel Bay, 35–45 minutes via the Broken Bay entrance) or from Middle Harbour/Rose Bay (60–70 minutes). The traditional marks put the Bahai Temple dome above the right-hand side of the Mona Vale Hospital tower with Barrenjoey behind Bangalley Head, but GPS is now essential.

The barge's deck rises 5 m off the sand, so it is an easy sounder target; anchor directly on structure.

The Long Reef wreck ground is 4–5 km offshore in open water with no lee. Any southerly or north-easterly swell above about 1.5 m makes anchoring on the wreck and the long decompression hang unpleasant or unsafe; charters generally want under 1 m swell and under 15 knots. Surface current from the East Australian Current can run north–south across the site even when the bottom is still, so a drifting deco station or a strong anchor line is essential.

Visibility is best from late autumn through winter (15–25 m) when the water is clear and cool; summer brings warmer water but more plankton and thermoclines. Avoid the days after heavy rain when the Narrabeen and Hawkesbury outflows dirty the inshore water. Mornings before the sea breeze are calmest.

Descend to the deck at 46 m. Tour the bow winch and bollards, look down into the three holds (the ferro-cement yacht that once sat in the first hold has largely disintegrated), check the stern steering compartment and the below-deck space forward of it, then return to the shot. Keep the dive short; the sand is 51–54 m.

Decompression dive. Typical plans use trimix (e.g. 21/35) or air/EANx with 50% and/or 100% O2 deco gases; 20–25 minutes bottom time gives 30–50 minutes of decompression. Twin cylinders or CCR with bailout, reel/line for any penetration, DSMB and surface support boat mandatory.

Silt inside compartments; several fatalities have occurred on this wreck group, so treat penetrations as overhead dives.

No scheduled recreational charters. Sydney Dive Charters (Rose Bay/Manly) runs technical charters to the Trio on demand for T1/Tec45+ certified divers; otherwise private boats and technical dive clubs launching from Roseville, Akuna Bay or Rose Bay ramps. Expect a handful of visits per year.