Meet the site
Under the Tube takes its name from the waves that tube over the rocks at the north-east corner of Moon Island, and the dive is on the steep wall beneath them. Grey Nurse Charters describes a rocky reef wall from 12 to 22 m with the Slot Cave, transitioning to sand in the deeper sections, and notes a strong population of eastern blue devilfish. iNaturalist's Moon Island notes add a steep wall, the 'amazing' Slot Cave and the Donut Cave. The wall is the main attraction: sheer in places, with cracks, holes and overhangs that shelter a wide range of fish and invertebrates. The Slot Cave is a narrow cleft cutting into the rock - a spectacular feature in good visibility - while the Donut Cave is a rounded hole that frames divers nicely for photos. Soft corals, sponges and ascidians cover the shaded surfaces. Eastern blue devilfish are the resident stars. These shy, electric-blue fish with white spots hide in holes and crevices and are a favourite of photographers; Under the Tube has more of them than most sites in the region. Around them are bullseyes, one-spot pullers, red morwong, wrasse and wobbegongs, with weedy seadragons, bull rays and Port Jackson sharks recorded around the island. In winter, grey nurse sharks from The Temple sometimes move along the walls. At 12-22 m the dive suits Advanced divers, or Open Water divers with a guide, who are comfortable near a wall with some surge overhead. It pairs well with The Temple or The Arch on a double-dive morning from Swansea.
What you might see
Eastern blue devilfish (abundant), bullseyes, one-spot pullers, red morwong, wrasse, wobbegong sharks, weedy seadragons, bull rays, Port Jackson sharks, soft corals, sponges
Location
A steep rocky reef wall on the north-east corner of Moon Island, 12-22 m, featuring the Slot Cave and Donut Cave and a strong population of eastern blue devilfish.
Facilities nearby
Departs Swansea (Black Neds Bay ramp): parking, toilets; air fills in Swansea.
Before you make plans
Conditions, access and operator schedules can change. This is a location guide, not a dive plan. Ask a local operator about current conditions and suitability for your training and experience.
Sources & further reading
greynurse.com.auinaturalist.ala.org.augreynurse.com.auRecord updated 26 Sept 2026. Source claims are being reviewed.