'North Head Sponge Gardens' is the name Manly operators give to the central stretch of the inner North Head reef, between Old Man's Hat and the Waterfall, and it is used both for a specific anchorage and loosely for the whole sheltered face. Dive Centre Manly lists it at 10–26 m as an Open Water site; Sydney Dive Charters describes 'groves of colourful soft corals and sponges' at 16–24 m; Pro Dive lumps the area with Flagstaff, Old Man's Hat, North Head Wall and Runway 36 as the 'North Head Peninsula'. The structure is the usual North Head mix: a boulder-strewn slope from the cliff to a sand edge at 20–26 m, with short walls, gullies and overhangs, and a thermocline at about 15 m in summer.

What distinguishes this stretch is the density of the fixed life. Sydney Dive Charters counts more than 60 species of sponge and coral in the North Harbour reserve, and on the shaded faces here they form a continuous mosaic – finger, vase and encrusting sponges, gorgonian fans, sea whips, soft corals and bryozoans – with basket stars, nudibranchs and decorator crabs living on them. Weedy sea dragons are reliable along the sand line, wobbegongs and crested horn sharks lie under the ledges, Port Jackson sharks crowd in during winter, and bull rays, blue groper, yellowtail and old wives are always about.

Frogfish and anglerfish turn up for the sharp-eyed. The site is sheltered from the north-east and almost always diveable, which makes it the workhorse second dive of every Manly boat and a favourite for Open Water course ocean dives. Tidal current along the face can be enough for a gentle drift.

Its value is exactly that reliability: colourful, calm reef diving ten minutes from the ferry wharf, inside a reserve where nothing may be taken. It suits every level, and photographers who set up a macro lens rarely regret it.

Boat only. Dive Centre Manly loads at Manly Cove beach beside the wharf; Pro Dive Manly and Sydney Dive Charters load at the Little Manly boat ramp (end of Stuart Street, Manly – free parking, set-up benches, toilets at the reserve); Sydney Dive Charters also picks up at Rose Bay public wharf. From Manly the run is 10–15 minutes across North Harbour to the inside of North Head, 20–25 minutes from Rose Bay.

Private boats launch at Little Manly, Roseville Bridge (Middle Harbour, 30 minutes) or Rose Bay. Boats anchor on the reef in 15–20 m or run in toward the cliff and drop the anchor when the bottom appears; there are no moorings, and many abandoned anchors litter the reef.

The inner face of North Head is the most sheltered ocean-quality diving in Sydney. It faces west-south-west into the harbour, so it is protected from the north-east sea breeze and from northerly and easterly swell, and is at its best when a strong north-easter is blowing and every ocean site is rough. It is exposed to southerly swell rolling through the Heads: a big southerly (over 2 m) sends surge along the cliff and dirties the water, though the deeper sponge gardens are usually still diveable.

Tidal current runs along the wall, east on the flood and west on the ebb, strong enough to plan a drift; slack water is easiest for novices. Visibility is 10–15 m on average, 25 m on winter westerlies, and drops after heavy rain when the harbour flushes brown. Ferry, shipping and small-boat traffic through the Heads is constant – ascend on the anchor line or a DSMB close to the cliff.

Descend to the reef at 16–18 m and let the tide set the direction: follow the sand edge at 20–24 m for 10–12 minutes, then rise to 14–16 m and return along the walls and boulder faces where the sponges are thickest. Stay on the reef to the anchor and ascend the line with a safety stop at 5 m.

Dive Centre Manly – double boat dives most weekends and some weekdays from Manly Cove (10–15 minutes); Pro Dive Manly – weekend double boat dives from Little Manly; Sydney Dive Charters – guided double dives Thursdays and Sundays from Little Manly ramp and Rose Bay wharf; Frog Dive, Abyss and Dive 2000 on club/charter days. North Head is the default Sydney boat dive when ocean sites are blown out.