The Dee Why is the largest and best known of the vessels scuttled to create the Long Reef artificial reef, and for three decades it has been the benchmark deep wreck for Sydney technical divers. Built by Napier and Miller at Glasgow and launched on 27 February 1928, she was a 67 m (220 ft), 799-ton double-ended steam ferry with a triple-expansion engine and twin screws, licensed to carry 2,000 passengers on the Circular Quay to Manly run. She worked the harbour for forty years until withdrawn in July 1968.

Stripped of her superstructure and engine, she was towed out and her sea-cocks opened on 25 May 1976; she went down in about 45 minutes and settled upright on the sand 4.25 km off Narrabeen with her bow to the east. Today the hull sits in 48 m with the highest remaining structure at about 43 m. The wooden main and promenade decks have gone, so the ferry now reads as a long open steel trough: the ribs and side plating stand, the bow end has folded to port, and the interior is dominated by four enormous boilers that survived the stripping.

Divers swim between and over the boilers and follow the propeller shaft tunnel, which is visible inside and outside the hull. Toilets at the eastern end, hold spaces and the remains of the double-ended steering gear are the other landmarks. Compared with photographs from the 1990s the wreck has slumped considerably, probably from anchors dragging through the structure, and the once-clean lines are now a maze of bent frames, so orientation on a poor-visibility day is harder than the size of the wreck suggests.

Fish life is good rather than spectacular: schools of nannygai and bullseyes in the shadows, kingfish over the top, Port Jackson sharks in winter and the occasional jewfish. Bronze and dusky whalers are regularly seen on the decompression stops and are the reason most divers hang in a group. The Meggol lies about 20 m off the starboard side near the bow, and the Coolooli, Himma and the barges are all within a few hundred metres, so a well-equipped team can visit two wrecks in one deco dive.

This is a dive for trained decompression divers with cold-water experience. It matters locally because it is the most intact large wreck close to Sydney at a depth that is reachable on a single-stage trimix plan, and because it is the anchor of the only designated artificial reef ground on the northern coast.

Boat only. Charters and private boats usually launch from the Roseville Bridge ramp on Middle Harbour or Rose Bay/Manly in Sydney Harbour and run north up the coast past North Head, Manly and Dee Why (roughly 45–60 minutes at cruising speed), or from the Akuna Bay/Pittwater ramps and run south past Barrenjoey (similar time). There is no all-weather ramp on the Northern Beaches ocean coast itself.

The wrecks lie on a flat sand plain and cannot be seen from the surface; you need accurate GPS and a sounder, and must drop the anchor or shot line directly onto structure because it will not hold in the sand. A drifting decompression station with spare gas is the normal arrangement.

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 the shot to the promenade deck level at about 43 m and clip a strobe to the line. Work the length of the hull from the eastern (bow) end west along the main deck, dropping to 46 m to pass through the boiler room swim-throughs between the four Scotch boilers, then out along the shaft alley if trained. Turn at two-thirds of planned bottom gas, return to the shot, and complete staged decompression on a drifting or fixed station with the 6 m stop where dusky whalers sometimes circle.

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 Long Reef wreck area 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.