Sinker Reef is the third member of the Hardy–Line–Sinker complex on the outer reef east of the Whitsundays. 100 Magic Miles describes a reef top covered in staghorn coral, with steep walls featuring ledges and caves, depths of 5–30 m and visibility of 8–18 m. Its fish list mirrors Line Reef's: cod, sweetlips, wrasse, coral trout and angelfish. The walls are the main dive, and the ledges and caves add interest as divers move along them.

The staghorn-covered top is attractive but lies in the current path, and the guide recommends against snorkelling because of it. Currents should be watched carefully and are similar to those at Line Reef; drift diving with an SMB and live boat is the safest approach. Depths to 30 m and strong currents make Sinker Reef a site for Advanced divers.

The reef's position between Hardy and Line puts it in the path of tidal water flowing through the gaps between the reefs, which is what supports its fish life but also demands careful tide planning. Sinker Reef is not visited by scheduled day trips; liveaboards and private charters are the only realistic access. The name should not be confused with other reefs of the same name elsewhere on the Queensland coast.

Current coral condition is not documented in the sources reviewed, and coordinates here are approximate. As with Line Reef, the dive rewards divers who relax into the current, stay close to the wall and let the boat do the work of collecting them at the end; fighting the flow wastes gas and misses the fish sheltering in the ledges.

Boat only. Hardy Reef lies on the outer Great Barrier Reef roughly 80 km offshore from Airlie Beach and Hamilton Island. Cruise Whitsundays' high-speed catamarans depart Port of Airlie (Airlie Beach) daily, with connections from Hamilton Island and Daydream Island, and reach the Reefworld pontoon in about 2 hours; scenic flights (about 30 minutes) also land at the reef by seaplane or helicopter.

Paid parking is available at Port of Airlie. Divers book certified or introductory dives on board. Other Hardy sites and the neighbouring Line, Sinker and Fairey reefs are reached by liveaboards and private charters; private vessels must use public moorings and observe no-anchoring areas.

Hardy and its neighbours are outer-shelf reefs fully exposed to the south-east trades (15–25 knots April–September). The large Reefworld catamarans run in most weather, but the crossing is rough in strong winds and diving can be restricted; smaller charter boats need a settled forecast. Hardy Reef's lagoon drains through a deep channel (about 60 m) beside the pontoon, and strong tidal currents run along the reef edges and through the gaps between Hardy, Line, Sinker and Fairey reefs, especially on spring tides and at the bottom of the tide – plan edge and wall dives for slack water.

Visibility is typically 8–20 m, up to 30 m on the outer faces. Marine stinger season (box jellyfish and Irukandji) runs roughly November to May – wear a full-length stinger suit or wetsuit with hood and gloves, carry vinegar. Cyclone season November–April; tropical lows and cyclones can close sites and damage coral (TC Yasi 2011, TC Debbie 2017).

Descend the wall to 15–25 m, drift along it examining ledges and caves, and ascend to the staghorn-covered reef top for the safety stop. Watch the current carefully.

No regular day-trip operator; visited on liveaboard/overnight dive trips from Airlie Beach and private charters. Cruise Whitsundays' Reefworld is nearby on Hardy Reef.