Barrenjoey is the sandstone headland, crowned by its 1881 lighthouse, that ends the Northern Beaches and guards the southern side of Broken Bay, and its underwater slopes are the least-dived good reef in the Sydney region. Michael McFadyen's Sydney index lists a site here, 'Dragon Alley', without a detail page, and the name tells the story: gutters between the kelp-covered boulders where weedy sea dragons are found. The headland's ocean face falls as tumbled boulders from the cliff to sand at 18–22 m; the northern face inside the bay entrance is gentler, with ledges and rock platforms in 6–15 m, and the western (Pittwater) side shoals into seagrass.

Marine life reflects its position between the harbour-like waters of Broken Bay and the open coast: blue groper, wobbegongs and Port Jackson sharks under the ledges, sea dragons in the kelp, bullseyes and nannygai in the crevices, cuttlefish, red morwong, bream and luderick over the reef, kingfish and salmon running the point in season, and eagle and bull rays on the sand. The shaded boulder faces carry sponges and sea tulips, though not in North Head's density. Visibility is the variable, swinging from 15 m in winter to almost nothing after a Hawkesbury flood, and the tidal stream around the point is strong enough to make it a drift on springs.

Because there is no dive shop north of Mona Vale and no scheduled charter, the site is the province of Pittwater boat owners and clubs, usually combined with the Valiant tug 1.4 km to the east. It suits Open Water divers with boat support in calm weather and Advanced divers when the tide is running. Depths, structure and the precise location of Dragon Alley are not published and are given here from regional knowledge; confidence is low.

Boat only. Launch at the Pittwater ramps – Careel Bay (Avalon), Bayview or Rowland Reserve – and run 15–25 minutes north past Palm Beach wharf to the headland; Sydney Dive Charters can pick up at Careel Bay Marina for a northern day. The headland's north-eastern and northern faces below the lighthouse have the best boulders; anchor where the sounder shows rock in 10–15 m or drift with a following boat.

Tidal streams in and out of Broken Bay run around the point. The Valiant wreck is 1.4 km east and is the usual second dive.

Barrenjoey stands at the mouth of Broken Bay and takes swell from north through east; the ocean face is only diveable in under 1.2 m swell, while the Broken Bay (northern and western) faces are more sheltered and are the fallback in an easterly. Tidal streams round the point are strong on spring tides – plan for slack water or dive as a drift. Hawkesbury flood plumes turn the water brown for a week after heavy rain; the best visibility (15 m) comes on winter westerlies at the top of the flood.

Anchor on the north-eastern corner in 12–15 m and work along the base of the boulder slope at 16–20 m with the headland on your left, checking the kelp gutters for sea dragons and the ledges for wobbegongs and groper, then return shallower at 8–10 m through the boulders. On a running tide, drift with the flow and surface on a DSMB for the boat.

No regular operator; Sydney Dive Charters northern charters on request (Careel Bay pick-up), Central Coast operators occasionally, otherwise Pittwater private boats and clubs.