Middleton Reef is the northern partner of Elizabeth Reef, about 230 km from Lord Howe Island across a 47 km-wide oceanic pass from Elizabeth, and one of the southernmost coral reefs in the world. It measures about 8.9 km by 6.3 km, and at high tide only a single sand cay about 100 m by 70 m remains above water. Like Elizabeth, it lies in the northernmost National Park Zone of the Commonwealth Lord Howe Marine Park and is listed as a Ramsar wetland of international significance. Parks Australia notes black cod, green turtles and Galapagos sharks among its protected species and describes diving and snorkelling here as a once-in-a-lifetime experience.

The reef's surface is dominated by shipwrecks. The largest is the Runic, a 13,500-ton ship (SS Runic, built 1949) that ran aground in 1961 and burned; it is now a broken, charred hulk stripped of fittings and is listed as its own entry. The Japanese tuna boat Fuku Maru (1963) lies high and dry, burnt out and in very poor condition, at about the four o'clock position on the outer reef, and the British barque Annasona (1907) was once a landmark and was used by the RAAF for bombing practice in World War II. The combined list for the two reefs runs to 26 named wrecks and 12 unidentified sites.

That wartime use has a modern consequence: Parks Australia has declared a 500 m exclusion zone around suspected unexploded ordnance at Middleton Reef, and the area is closed to visitors. Divers must check the current notice before planning.

Recreational fishing is prohibited at Middleton Reef; diving and snorkelling are permitted without a permit. There are no facilities, and reaching it requires an ocean-going vessel and a settled weather window. It is a destination for experienced expedition divers only, and depths and specific dive sites are not published.

Elizabeth and Middleton Reefs are remote, uninhabited coral reefs in the Tasman Sea, about 150 km (Elizabeth) and 230 km (Middleton) north of Lord Howe Island and 555–600 km off the NSW coast. There are no facilities, moorings or regular dive trips. Access is by private ocean-going yacht, research vessel or occasional expedition charter departing Lord Howe or the NSW coast, typically a one- to two-day passage each way.

Anchoring is possible in the lagoon in suitable weather; Elizabeth has a north-east entrance historically recorded with about 7 fathoms over the bar at low tide.

The reefs are fully exposed to the open Tasman Sea. Passage and diving require a settled window of light winds and small swell; any sizeable south-east trade wind swell or a southerly change makes the outer reef undiveable and the lagoon anchorage uncomfortable. Late summer and autumn (February–May) are the likeliest months for calm windows; winter brings fronts and big seas.

Tropical cyclones and ex-tropical lows can track through between December and April. Visibility on the outer reef is often 30 m or more.

Anchor in the lagoon in settled weather and dive the lagoon bommies and the leeward outer slope from a tender; keep well clear of the 500 m ordnance exclusion zone.

No regular operator – private ocean-going yachts, research vessels and occasional expedition charters only.