Northern Outer Reef is one of the Dive Shack's listed Adelaide metropolitan boat sites, and it is one of the least documented. It is described as a section of ancient shoreline about 100 m long, running north to south, in 18 m of water. Like Mac's Ground, Milkies and Seacliff, it is a remnant of the coastline that existed when sea level was much lower, now forming a low ledge on the sandy floor of Gulf St Vincent.

The Dive Shack highlights vast amounts of sea sponges on the reef, which is typical of Adelaide's deeper offshore ledges where there is less light for algae and more room for filter feeders. Young blue devils are noted in the holes, and the reef has overhangs along its length. Beyond that, expect the usual offshore Adelaide reef fish – old wives, bullseyes, leatherjackets, magpie perch and wrasse – plus invertebrates such as nudibranchs and spider crabs on the sponge-covered rock.

At 18 m, the site suits Open Water divers with some offshore experience and Advanced divers. It is compact enough to see in a single dive, and the north-south orientation makes navigation straightforward once the reef is found. The main challenge is locating it: no public position is published, so arrange a mark with the Dive Shack or a club before heading out.

Divers who visit are encouraged to share observations with the Scuba Divers Federation of SA so the record can be improved.

Boat access only. The Dive Shack lists the site as offshore Adelaide without a published position; divers will need a GPS mark from a local shop or club. Launch at West Beach or Glenelg (Holdfast Shores) ramps and run offshore to the mark.

There is no mooring – anchor on sand beside the reef and leave an attendant on board with a dive flag.

An offshore Gulf St Vincent site, diveable only in light winds, ideally easterlies (offshore) on summer and autumn mornings before the south-westerly sea breeze. Westerly and south-westerly winds and swell make it uncomfortable and reduce visibility. Tidal movement is small but can be felt on spring tides; neap dodge tides are best.

Visibility is usually better than on the inshore reefs.

Anchor at one end of the 100 m reef section, swim its length at 17-18 m looking under the overhangs, then return along the other side to the anchor.

No regular operator; listed by The Dive Shack (Glenelg) as a local boat site – enquire with the shop for trips or the GPS mark.