Tongue Reef is a large, intricate reef complex east of Port Douglas. Wavelength describes the route to Opal Reef as passing through the protected Tongue Reef maze, and its snorkel sites at Third and Fourth Sister reefs lie just off Tongue Reef's seaward side, around 90 minutes from Port Douglas. The maze character comes from numerous reef patches separated by channels, which boats must navigate carefully, particularly at low tide.

Much of the complex is protected, and it is not a regular mooring area for the big day boats. The reefs that operators do use sit on the outer (seaward) edge, where water exchange with the Coral Sea keeps visibility and coral health high. Wavelength's accounts of the Sister reefs describe very good coral cover, from large boulder corals to branching coral patches and huge giant clams, with resident green turtles, grey reef sharks and large barracuda, plus occasional hammerheads, tiger sharks and sea snakes around the major reef gap.

For divers, Tongue Reef is essentially a region rather than a single site. Its sheltered channels have potential for exploration on private charters, but the published information covers the Sister reef moorings. If you want to experience this part of the reef, the easiest way is a Wavelength snorkel day; scuba divers should talk to a small-group operator about a charter to the Sister reefs.

Wavelength Reef Cruises departs Crystalbrook Superyacht Marina, Wharf Street, Port Douglas, at 8:00 am and returns about 4:30 pm; park at the marina or on surrounding streets. The Sister reefs lie just off the seaward side of Tongue Reef and the crossing takes about 90 minutes, threading through the Tongue Reef maze. Wavelength is snorkel-only and holds exclusive moorings here; snorkellers enter from the stern platform at the mooring.

Scuba access requires a private charter with a small-group dive operator.

The Sister reefs sit on the seaward side of Tongue Reef close to the shelf edge and are exposed to the south-east trades that blow from about April to September, so moorings are chosen to suit the wind and large swells rule out the exposed sites; a 25-knot-plus trade wind makes the crossing rough and operators may cancel. Exposed sites on the seaward face and in the channels through the major reef gaps are dived only in light winds and suitable tides. October–December is typically calmest with the best visibility; the wet season (December–April) brings northerlies, storms, cyclone risk and marine stingers (November–May).

Tidal currents run through the gaps between the Opal reef sections and around bommies, strongest on spring tides – channel and bommie sites are timed for neaps or slack water.

Tongue Reef itself is mostly passed through rather than dived; snorkel or dive trips use the named moorings on the adjoining Sister reefs (see separate entries), following the guide from the boat along the reef edge.

Wavelength Reef Cruises – snorkel-only full-day trips with exclusive (low-impact) moorings at Third and Fourth Sister; scuba only by private charter (Scuba IQ / Blue Dive, Tech Dive Academy).