Nine Mile Reef sits about nine nautical miles offshore from the Tweed River/Gold Coast Seaway, far enough out to escape most of the inshore turbidity but exposed to the full force of the East Australian Current and open-ocean swell, which is why it is reserved for experienced divers. The reef is a low, broken system of ledges, boulder gutters and bommies rising from a sand bottom around 30 m to within 12 m of the surface, and its centrepiece is a natural bowl in the rock that local divers and operators call the Shark Pit. From roughly June to October this amphitheatre and the surrounding gutters hold one of the more reliable winter grey nurse shark aggregations on the southern Queensland/northern NSW border, sometimes with dozens of animals hanging motionless in the current at once — an experience that has made Nine Mile one of the better-known shark dives in the region, spoken of in the same breath as the Tweed/Gold Coast's other aggregation sites.

The sharks are generally placid if divers keep low, move slowly and do not crowd the gutter entrances. Outside shark season the reef still fishes and dives well: leopard (zebra) sharks lie on the sand in summer, wobbegongs pack into the ledges, giant Queensland gropers and bamboo sharks are seen in the deeper cracks, eagle rays and wedgefish cruise over the bommies, and big schools of trevally, kingfish and bonito move through on the current. The site's difficulty comes almost entirely from its exposure: currents of up to 2–3 knots are common, there is no shelter from swell, and a negative or shot-line entry followed by a mid-water or bottom-current fight is standard rather than exceptional.

Most operators will only run the trip in a narrow weather window, and it is common for bookings to be moved or cancelled at short notice through the cooler months when the shark numbers are highest but the weather is least reliable. Because of the distance, current and depth, Nine Mile is not a beginner's dive; it suits Advanced Open Water divers with genuine drift-diving and current experience who want a real chance of a big grey nurse shark encounter without a boat trip to more remote NSW aggregation sites.

Charters leave from Tweed Heads (Tweed River, near the Twin Towns end) or Point Danger/Kirra ramps on the Gold Coast side; most Tweed-area shark and reef trips are actually run out of Gold Coast Dive Adventures at Currumbin or Kirra Dive at Coolangatta, a short drive/paddle across the border from Tweed Heads. Private boats launch at the Jack Evans Boat Harbour ramp on the Tweed River (Wharf St, Tweed Heads) or Boyds Bay ramp, then run out across the Tweed River bar — a shallow, occasionally rough bar that should only be crossed at higher tide in settled weather. From Coolangatta/Currumbin it is roughly a 30–45 minute run east; from the Tweed River it is a similar distance and time once past the bar.

Trips are frequently rescheduled around swell and current, so operators keep a shorter list of backup days.

The Tweed/Coolangatta coast is exposed to south-easterly and easterly swell year-round; charters generally want under 1.5 m swell and light winds. A strong East Australian Current push (common November–May) can add 1–2 knots of southerly drift on the outer reefs, turning some sites into serious drift dives. The Tweed River bar and river mouth are tide- and swell-dependent and can close to small boats in a big easterly swell.

Visibility on the inshore reefs drops for days after heavy rain flushes the Tweed River; the offshore reef and shark sites are less affected. Winter (June–September) gives the clearest, calmest water and the grey nurse shark season; summer is warmer with more current and better odds of leopard sharks and mantas. Being fully exposed and 15 km offshore, Nine Mile is cancelled well before the inshore reefs; operators generally want under 1 m swell and light winds before committing a boat.

Descend the shot line directly into the Shark Pit gutter at 18–22 m, hold position quietly at the edge of the amphitheatre to let the resident grey nurse sharks settle back in, then work along the ledge system to the shallower bommies at 12–15 m for the ascent, keeping a close eye on the current and staying near the group.

Gold Coast Dive Adventures (Currumbin) runs dedicated Nine Mile shark trips through winter, weather permitting; Kirra Dive (Coolangatta) and occasional Tweed-based charters also visit. Most operators require 20+ logged dives or an Advanced certification before booking this site.