The Port Noarlunga underwater trail was one of the first interpretive dive trails in Australia. Installed in 1994 within what was then the Port Noarlunga Reef Aquatic Reserve, it consists of 12 concrete plinths set on the seabed over about 800 m, each carrying a plaque that describes the plants and animals of the reef. Seven markers are on the sheltered inside of the reef, marker 8 is in the Gap that divides the northern and southern sections, and the last four are on the more exposed outside.

Following the trail is the classic way to learn Port Noarlunga. The inside markers run along the boulder-strewn lagoon in 4-6 m, past overhangs where old wives, moonlighters, zebrafish and magpie perch shelter and where Port Jackson sharks lay eggs in early summer. Plaques point out sponges, ascidians, bryozoans, crustose coralline algae and the reef's many molluscs, making it a good introduction to temperate reef ecology.

Nudibranch enthusiasts will find plenty of subjects along the way. The Gap is the trail's pivot and its challenge. It drops to around 20 m and the current can be strong when the tidal range exceeds about 1 m, so continuing to the outside markers is a job for experienced divers on slack water.

The outside is more open, with larger swells, more water movement and often schools of fish such as sweep and mackerel. The trail suits newly certified divers building navigation skills, snorkellers (for the inside markers at high tide), and families wanting an educational swim. Some plaques are weathered and encrusted after three decades, but the plinths remain useful waypoints.

Treat it as a structured version of the Port Noarlunga Reef dive rather than a separate site.

Drive to Port Noarlunga (35-40 minutes south of Adelaide), park at the jetty base or the larger car park 100 m away, and walk the 300 m jetty to the end stairs, which enter in about 6 m right beside the reef. The trail starts close to the jetty on the inside of the reef. A trolley helps with gear on the jetty.

Snorkellers can follow the inside markers from the surface at high tide.

The reef runs parallel to the shore and shelters the inside lagoon from the prevailing westerly and south-westerly swell, so the inside can be dived when the outer reef is rough; even so, strong westerlies or any swell over about 1 m make the beach entry surfy and the water murky. Best conditions come after several days of offshore easterly winds, on summer and autumn mornings before the south-westerly sea breeze. Tide matters: local guides warn of strong current when the difference between high and low tide exceeds about 1 m, especially through the Gap, so plan around high slack water and prefer neap (dodge tide) days.

Visibility is poorer after rain, when the Onkaparinga River discharges just south of the reef.

Descend from the end stairs and follow markers 1-7 south along the inside of the reef at about 5 m, reading the plaques. Marker 8 sits in the Gap (deeper, with current); only continue through to markers 9-12 on the outside at high slack water and with gas in hand. Either return the same way or, for a full circuit, come back along the outside and re-enter the lagoon via the Gap before finishing at the jetty.

Diving Adelaide, Adelaide Scuba, Underwater Sports and The Dive Shack use the trail for guided dives and navigation training; also dived independently every day in summer.