Ashmore Reef lies in the Timor Sea about 320 km off the Kimberley coast, in the Commonwealth Territory of Ashmore and Cartier Islands. It is a biodiversity hotspot with about 833 fish species, 433 molluscs and 178 echinoderms recorded, around 100,000 breeding seabirds, and green turtle nesting; shark numbers have recovered significantly since 2004. It was historically famous for sea snake diversity.

The reef lies within the 1974 Australia–Indonesia MOU Box, where traditional Indonesian fishers have access, and divingwawrecks notes numerous small Indonesian perahu sunk by authorities for illegal fishing or people smuggling, scattered through the shallows. Both Ashmore Reef and Cartier Island marine parks are strict nature reserves except a recreational use zone around West Island; entry to sanctuary zones requires approval.

Kimberley diving context: the Kimberley coast between Broome and Wyndham is one of the least-dived coastlines in Australia. Huge tides (up to 10–11 m), turbid inshore water, saltwater crocodiles around the islands and rivers, and large sharks mean that inshore diving is generally discouraged by experienced local boaters, and expedition cruise ships mostly offer snorkelling or Zodiac excursions rather than scuba. The serious diving is offshore, on atolls and island reefs far from river run-off, visited on a handful of liveaboard departures a year from Broome or Darwin. Medical help is many hours away; divers should be conservative and carry DAN or equivalent evacuation cover.

Planning notes: Advanced Open Water certification is recommended and the site is rated advanced. Expect to spend most of the dive around 12 m, with a maximum of about 30 m. It can only be reached on a multi-day expedition or liveaboard, so it is usually one of several sites dived on the trip and choices are made by the crew on the day. Currents are significant, so timing around the tide is part of the plan. Best season: September–November (calmest window). Traffic: remote/expedition only.

Only by expedition vessel from Broome or Darwin. Ashmore Reef Marine Park is managed by Parks Australia as a strict nature reserve except for a recreational use zone at West Island; vessels need prior approval to enter sanctuary zones. Cartier Island has unexploded ordnance and is closed.

Expedition vessels typically depart Broome or Darwin (or Wyndham) on 10–14 night Kimberley voyages, or on dedicated 7–12 night reef expeditions; private vessels need full offshore capability, long-range communications (satphone/EPIRB) and fuel for several hundred nautical miles. Dives are run from tenders launched from the mothership, which anchors in the lee of the reef.

Kimberley offshore trips run in the dry season (roughly April–October; Scott Reef and Rowley trips cluster September–November when winds ease). The wet season (November–April) brings cyclones, and the Kimberley coast has tides up to 10–11 m, creating violent currents, whirlpools and tidal races near the mainland and inshore reefs. Offshore atolls have smaller ranges but strong currents in channels.

Strong easterly trade winds in mid-dry season can make crossings rough.

Only within approved areas (West Island recreational zone): dive the reef slope from a tender.

Rare expedition voyages only, with Parks Australia approval.