Tusa Canyons is named after Tusa Reef Tours, the Cairns operator that pioneered the site, and Cairns Dive Adventures lists it with Cathedral as one of Thetford Reef's two famous sites. Thetford's structure is a field of scattered coral bommies rising from sand at around 25 m, and at Tusa Canyons the bommies are close enough together that the sand channels between them form canyons: narrow, steep-sided corridors with coral walls, overhangs and occasional swim-throughs. Swimming the canyons gives the dive a sense of exploration, and the sandy floor bounces light up onto the walls so the colours are bright.

Dive the World's description of Thetford applies well here: giant clams several metres long, stingrays and turtles, gorgonian fans with tiny critters, Napoleon wrasse, moray eels, octopus and Christmas tree worms, along with anemones and anemonefish, trevally, emperors, red bass and fusiliers. Coral trout and cuttlefish are common, and seahorses and nudibranchs reward those who search the fans and rubble. The canyon walls also make natural cleaning stations.

Current can funnel through the channels on a running tide, which keeps the coral healthy but means divers should stay together and follow the guide's route. The maximum depth of about 25 m is only reached at the base of the deeper bommies, so most of the dive is at comfortable Open Water depths. Specific published descriptions of this mooring are limited, so some detail is drawn from Thetford Reef summaries.

The site is typically dived as part of a two- or three-site day on Thetford, often following Cathedral, and it is a good choice for divers who enjoy navigating interesting terrain.

Boat access only. Cairns day boats leave from the Reef Fleet Terminal (1 Spence Street) and the neighbouring Marlin Marina on the city waterfront, with check-in between about 7:00 and 8:00 am; most operators run transfers from city and northern-beaches accommodation, and there is paid parking near the terminal. The run of about 45 km out through Trinity Inlet and across the lagoon to Thetford Reef takes around 80 minutes on a fast catamaran and noticeably longer on slower dive boats or into a south-easterly chop.

Tusa Canyons takes its name from Tusa Reef Tours and is also visited by Silverswift and the Pro Dive Cairns liveaboard. Enter from the stern platform.

The prevailing south-east trade winds (typically 15-25 knots from April to September) are what limit Cairns outer-reef diving: above about 25 knots trips may be cancelled, and exposed eastern moorings are used only in light winds. Summer northerlies are gentler but make north-facing moorings choppy. Box jellyfish and Irukandji are present from about November to May (stinger suits on every dive), and the cyclone season runs November to April, when a system in the Coral Sea can close the reef for days.

Visibility drops after heavy rain and strong winds; spring is usually calmest and clearest. The canyons are fairly sheltered in trade winds but can channel tidal current; visibility falls after heavy rain as Thetford is nearer the coast than Flynn or Milln.

Descend to the sand at 12-15 m and follow one canyon between the bommies, looking into the overhangs and undercuts, then cross over a saddle into the next canyon for the return. Keep an eye on current direction in the channels. Finish on a bommie top with a safety stop at 5 m.

Silverswift – regular day trips (Thetford is one of its four core reefs); Passions of Paradise – day trips to 360 and East Meets West; Pro Dive Cairns – 3-day liveaboard includes Thetford; Tusa Reef Tours – occasional