The Cod Hole is one of the two or three sites every diver who visits Julian Rocks wants to do, a genuine swim-through or arch cut through the rock at the northern/eastern end of the island where the more exposed, current-swept side of the reef begins. The entrance sits around 15 to 18 m and the passage slopes down through the rock to a sandy floor at 20 to 22 m, wide and tall enough to swim through in comfort rather than squeeze through, which is what makes it accessible to Advanced Open Water divers rather than being reserved for technical specialists. Grey nurse sharks are the headline attraction from roughly May to October, when a rotating population of these placid, toothy-looking but harmless sharks uses the cavern and the surrounding walls as a daytime resting spot, hanging motionless in the current with their mouths open in a way that unsettles new divers until they realise the sharks have no interest in them at all.

Even outside shark season the swim-through and the sand beyond it are consistently good for large animal encounters: moray eels sit in cracks along the walls, big blue gropers cruise past, schools of bullseyes and kingfish move through the passage, and the sandy bottom beyond the cave regularly holds larger pelagic visitors working the current line. Turtles are common on the approach and departure. Because it sits on the more exposed side of Julian Rocks, the Cod Hole cannot always be run — a strong easterly or southerly swell will push too much surge through the passage to dive it safely, and operators will substitute the Nursery or Hugo's Trench on those days.

When conditions are right, though, this is regarded as one of the best temperate/subtropical swim-through dives on the entire east coast, combining genuine cavern topography with one of the most reliable grey nurse shark encounters south of the tropics, and it is the dive most repeat Julian Rocks visitors specifically request.

Both Byron Bay dive operators launch small rigid-hull inflatables directly off the beach: Sundive Byron Bay launches from The Pass at the northern end of Main Beach, and Byron Bay Dive Centre launches from the beach near the Wategos/Clarkes Beach end; check in first at the shopfront in town (Sundive: Shop 8, 9-11 Byron St; Byron Bay Dive Centre: Byron Plaza, 5/90-96 Jonson St), then walk down to the beach for the boat launch briefing. It is a short, often bumpy, beach-launch surf crossing before a 5-10 minute run out to Julian Rocks, so loose gear should be secured. No private-boat ramp serves the rocks directly; private boats normally launch from the Brunswick Heads river mouth or Ballina and run up or down the coast.

Julian Rocks sits fully exposed 2.5 km off Byron Bay, so conditions depend on the swell and wind direction rather than any tide window. Trips are cancelled or moved between the Nursery (sheltered west side) and the more exposed east/south sides depending on swell direction and size; a big easterly or south-easterly swell above about 2 m closes the outer sites but the Nursery can often still run. Visibility is genuinely tide- and current-driven here as warm East Australian Current water and cooler southern water meet around the rocks — it can change from 5 m to 20 m within a single day and even between two dives an hour apart.

Summer (Dec-Mar) is warmest (21-26°C) with the leopard shark aggregation and manta ray sightings; winter (Jun-Aug) is cooler (18-21°C) but usually clearer, with grey nurse sharks and migrating humpback whales visible from the boat. Heavy rain from the Brunswick and Belongil systems can dirty the inshore approach for a day or two but rarely affects the rocks themselves once you are out.

Descend on the mooring to about 15 m at the cave/arch entrance, pass slowly through the swim-through watching for grey nurse sharks resting in the lee of the walls, continue to the sandy bottom at 20-22 m where large pelagics congregate, then return over the top of the structure for the ascent.

Sundive Byron Bay (The Pass) – daily double and triple boat dives; Byron Bay Dive Centre (Wategos/Jonson St) – daily double boat dives, Mon & Thu-Sun.