One Tree Island Reef is one of the most scientifically important reefs on the Great Barrier Reef, but it is not a recreational dive site. The reef, about 5.5 km by 3.5 km with a rubble and shingle cay at its eastern end, lies about 96 km north-east of Gladstone and is home to the One Tree Island Research Station, managed by the University of Sydney since 1974. The reef sits within a Scientific Research (Orange) Zone, one of only two such no-entry zones in the entire Marine Park (the other is near the Australian Institute of Marine Science off Townsville).

The zone exists so that long-term research and monitoring can be carried out without disturbance, and compliance officers have publicly reported recreational vessels detected transiting it without permission. Research at One Tree has produced more than 400 published papers on climate change, coral bleaching and reef ecosystems, and the reef's shallow rhodolith beds and enclosed lagoons make it a natural laboratory. The station hosts educational groups of up to about 30 senior high school and university students for field training, which is the only practical way for most people to visit.

For divers, the relevance of One Tree is as context: the healthy condition of the surrounding Capricorn reefs is monitored here, and much of what is known about reef responses to warming and storms in this region comes from this station. It is included in this database because it appeared in the first pass as a dive site; it should be treated as closed. Divers wanting similar remote Capricorn reef experiences should look at Sykes, Fitzroy or Lamont reefs instead.

One Tree Island is about 96 km north-east of Gladstone and south-east of Heron Island. The reef is a Scientific Research (Orange) Zone: recreational vessels may not enter without GBRMPA/QPWS authorisation, and boats have been reported for transiting it. Access is limited to researchers and approved university and school groups (up to about 30 students) via the University of Sydney One Tree Island Research Station.

Recreational divers should view it on the chart only and dive neighbouring reefs instead.

The Capricorn cays are exposed to the south-east trades (often 15-25 knots April-September), which make the Gladstone crossing rough and restrict diving to the lee (north-western) sides. The calmest windows are typically September-December and March-April. Northerlies in spring and summer reverse the sheltered sides.

Tidal currents run strongly around reef edges and channels on springs. Cyclone season November-April. Conditions are academic for recreational divers because entry is prohibited without authorisation; researchers work around the same trade-wind and tide constraints as the rest of the Capricorn Group.

Not applicable – recreational diving is not permitted. Research diving is conducted under the station's own dive safety plans.

No recreational operator – closed Scientific Research Zone; access only through the University of Sydney One Tree Island Research Station for approved research and education groups.