Big Seal Rock is the larger of the two islands that give Seal Rocks its name, and it holds one of the most significant grey nurse shark aggregations on the entire NSW coast: the Eastern Gutter, a wide channel at around 21 metres with a small cave cutting into the rock face, has recorded more than 40 individual sharks gathered at once, a scale that places it alongside Forster's Pinnacle and Fish Rock's gutters among the state's premier shark-viewing sites. The island's underwater profile extends well beyond that single gutter, however — the site's full depth range runs from shallow reef around eight metres to a wall dropping to 45 metres in places, giving experienced divers scope for a genuinely varied multi-level dive rather than a single fixed shark encounter. Around the sharks, the water column is busy with schools of kingfish, drummer and trevally circling the gutter and the surrounding structure, while the bottom holds turtles, wobbegongs, stingrays and Port Jackson sharks tucked among the rock.

Seal Rocks as a whole has been dived and explored by local operators for more than 40 years, and the surrounding area is described as holding hundreds if not thousands of smaller, less formally documented reef patches beyond the two named islands — Big Seal Rock itself, though, is the headline site, dived close to daily by Forster Dive Centre when conditions allow. Its combination of a large, reliable shark aggregation, genuine depth range and diversity of secondary marine life makes it one of the standout dives on the mid-north coast, suited to confident, experienced divers comfortable with the depth and the moderate current that builds around the exposed Sugarloaf Point headland.

Boats for the Seal Rocks sites either run the longer distance down from Forster/Tuncurry (about 45-60 minutes) or launch locally from the small boat ramp at Seal Rocks village itself, near Sugarloaf Point, cutting the run to Big and Little Seal Rock down to 10-20 minutes. Seal Rocks is a small, remote settlement reached via Lakes Way/Mungo Brush Road through Myall Lakes National Park; there is no dedicated dive shop in the village itself, so most Seal Rocks charter diving is booked through Forster Dive Centre, with divers sometimes meeting the boat locally rather than travelling from Forster.

Seal Rocks sits on an exposed headland with Sugarloaf Point taking the full force of easterly and south-easterly swell — trips are cancelled in anything beyond about 1.5 m swell, and the site is often diveable when more sheltered locations further north are blown out, or vice versa, so conditions should always be checked specifically for Seal Rocks rather than assumed from Forster. Visibility is typically 15-30 m and can be excellent in settled autumn and winter weather; it drops after the Myall Lakes and Wallis Lake systems discharge following heavy rain. Grey nurse sharks are present at almost any time of year here, with numbers varying seasonally, and the wrecks (Satara, Catterthun) require calm, low-current conditions for safe deep/technical diving.

Descend the mooring onto the Eastern Gutter at around 21 m, where the sharks typically hold, then work through the small cave cutting into the rock face and along the surrounding wall (down to 45 m on the deepest sections) as time and training allow, watching schools of kingfish, drummer and trevally circling overhead and checking the bottom for wobbegongs, stingrays and Port Jackson sharks.

Forster Dive Centre (Tuncurry) — regular boat trips to Seal Rocks, year-round, weather permitting.