Mackerel Boulder sits near the Needles on the current-exposed southern flank of Julian Rocks, and as its name suggests, it is valued specifically for the schooling pelagic fish — mackerel, kingfish and trevally among them — that hold station in the current around the boulder rather than for any particularly dramatic topography of its own. This is very much a big-fish, current-driven site rather than a macro or cave dive, and its character changes noticeably depending on how strongly the East Australian Current is running on a given day. On a good current day, divers can expect dense schools of pelagic fish stacked around the boulder, taking advantage of the current break it creates to feed and rest before continuing to move with the flow; leopard sharks and eagle rays are also seen on the sand nearby in the warmer months, drawn to the same current-swept sandy arena that makes the neighbouring Needles such a reliable big-animal site.

On slack or calm days the boulder is quieter, holding more of a resident reef-fish population without the dramatic schooling spectacle. Because the site depends so much on current for its best encounters, Mackerel Boulder is usually dived as a drift rather than from a fixed mooring, with divers entering upcurrent and working with the flow past the boulder and on toward the Needles, carrying a surface marker buoy in case the group becomes separated from the boat. This makes it a slightly more involved dive than the sheltered Nursery sites, suited to divers with some drift-diving experience, but the reward of holding station in a strong school of pelagics in clear blue water is one of the more memorable big-fish experiences Julian Rocks offers.

Both Byron Bay dive operators launch small rigid-hull inflatables directly off the beach: Sundive Byron Bay from The Pass, Byron Bay Dive Centre from near Wategos/Clarkes Beach; check in first at the shopfront in town, then walk down to the beach for the boat launch briefing.

Julian Rocks sits fully exposed 2.5 km off Byron Bay; a big easterly or south-easterly swell closes the outer sites but the Nursery side can often still run. Visibility can change from 5 m to 20 m within a single day as warm and cool currents meet around the rocks. Summer (Dec-Mar, 21-26°C) brings leopard sharks and mantas; winter (Jun-Aug, 18-21°C) is cooler but usually clearer, with grey nurse sharks and migrating humpback whales.

Position downcurrent of the boulder at 9-13 m and hold station to watch schooling pelagics rather than chasing them, then drift with the current toward the Needles for the remainder of the dive before ascending under an SMB if needed.

Sundive Byron Bay (The Pass) and Byron Bay Dive Centre (Wategos/Jonson St) both run daily boat dives to Julian Rocks.